The character that leaves and rejoins the party is not permanently missable. It might be tricky to figure out how to get them back, but there's no fail state.
You can and should do the first playthrough blind. Save the guide for NG+.
There is an optional party member that you can either recruit or fight based on which dialogue option you pick. You'll know it when you see it though, so it's easy to make the right choice.
There are 12 endings (13 in DS and subsequent rereleases). You can easily see all of them in just two playthroughs. Theoretically you could even do them all on the first playthrough, but it's much easier to do in NG+.
The only caveat is that you have to see them in order, you can't backtrack if you miss one, which is why I recommend starting with the final and true ending on your first playthrough, then do all the others on NG+. NG+ makes it pretty easy to speed through things as well, your second playthrough will be much shorter.
I respect the hell out of him for doing the best he could with very limited resources, difficult technical limitations, and an insane deadline. I just can't recommend playing that version today over a better alternative.
There are a lot of JRPGs from this era that I love dearly but would have a hard time recommending to anyone who didn't grow up on these kinds of games. Games that are slow, grindy, and mostly consist of clicking Attack every turn.
Chrono Trigger is the one exception I can recommend to anyone, and then say that if you liked this entry point then you can try some other JRPG classics.
Just note that the original SNES translation should be avoided, play a modern rerelease or a retranslation patch.
The original game (but not CoH) is cleverly designed to be entirely playable with just four inputs, all non-movement actions can be performed with two simultaneous inputs (jumps). So it's entirely playable on a dance pad that way. I haven't tried it myself, but I know it's a thing you can do, and there's footage out there of speedrunners doing dance pad runs.
Cadence of Hyrule - The original Crypt of the Necrodancer is one of my all-time favorites. CoH doesn't quite reach that incredibly high bar, but it's still an excellent game in its own right.
Metroid Dread - Hits all the highs of Super, but with greatly improved combat and bosses. So yes, I am calling this better than Super, you heard me.
Puyo Puyo Champions - Technically, this is not exclusive. However, I am counting it as such because the online playerbase is dead on every other platform. If you want to play the greatest competitive puzzle game ever made, the Switch version of Champions is really the only option.
It's not like Square Enix doesn't know how to make good turn-based games. They've been hitting it out of the park with their smaller budget projects like Bravely Default and Octopath Traveler. So I don't know why they've rejected it for FF, imagine what they could do with a big budget title if they tried.
I joke about how halfway through development, someone at Square Enix must've realized that Bravely Default was actually a good game, and thus too good for the FF name. So instead they had to throw darts at an English dictionary to rebrand it.
Boktai trilogy: Hideo Kojima's greatest masterpiece. First game's alright, second game is where it comes into its own. Note that you want the Solar Sensor hardware for the full experience, but emulating them is worth it over not playing them at all. And for the third game, you'd have to pick between original hardware or the translation patch anyway.
Castlevania: Aria of Sorrow - It's Castlevania. Also play Circle of the Moon and Harmony of Dissonance, but Aria is by far the best of the GBA installments.
Golden Sun 1/2: These games were way ahead of their time for how they designed a combat system that encourages you to use all of your tools and not just click basic Attack as if you gotta hoard your MP for a rainy day. Fantastic puzzles too.
Mother 3: Surely you have already heard of this game and do not need me to tell you to go play it. Have you not played it by now? Why not? Well, okay, if you haven't played Earthbound first, go do so, then play this.
Rhythm Tengoku: A wonderful game about pressing the A button. Sometimes you press the d-pad too. Translation patch.
Summon Night: Swordcraft Story 1/2: If you've ever played the classic 2D Tales games, these are excellent spiritual successors to those. There's a third game that's JP-only, translation patch is being worked on but it's been stuck in development hell for years...
Romhacks:
Celeste Mario's Zap & Dash (NES): SMB1 turned into a Metroidvania with Celeste mechanics ported in. I think what impresses me the most is that they got 4-directional scrolling into this engine.
Super Metroid and A Link to the Past Crossover Randomizer (SNES): It's an absolutely incredible technical feat that this even works. SM and ALttP smashed together into a single ROM, with a few doors that take you from one game to the other, then the item pools are shuffled together so you have to go back and forth to find one game's items in the other. Unfortunately because ALttP is a much bigger game with a lot more items it kinda overshadows SM, you may not find this to be as replayable as the standalone randos. But I recommend trying it once because it's just so cool the first time.
The King of Fighters 2002: KOF fans will tell you either 98 or 02 were the absolute pinnacle. I side with 02 because it has Kula in it. Also note that 98 and 02 both have updated rereleases with an extended roster and rebalancing, but those are Windows-only.
Puyo Puyo Tsu: 20th Anniversary is the peak of the series, but if you're on hardware that can't run DS or Wii, arcade Tsu is fine. AI is a lot weaker though, and the story mode just forces five colors and high gravity on later stages to compensate.
Puzzle Bobble 1/3: You've probably played some flash game clone of this. IMO I think 1 was best for its simplicity, I'm not as fond of the garbage patterns introduced in later titles in an effort to give characters some asymmetry. But PB1 does not have AI opponents, singleplayer is only the stage clear mode, so if you don't have a human to play with try PB3 for the next best thing.
Tetris: The Grand Master 1/2/3: The only good Tetris, do not @ me. Start with TGM2's Novice Mode, then once you can clear that go back to TGM1.
Twinkle Star Sprites: A versus shmup with a very unique format. Chaining enemies on your screen sends attacks to your opponent's screen. Hard to really explain, just give this a spin and feel it out for yourself. There are a lot of moving parts, screenwatching is vital, and feels like I've barely scratched the surface of the game's depth.
Vampire Savior: Aka Darkstalkers 3. This game is fast as hell and it's a blast. Like with any classic fighter, good luck keeping up with FightCade folks who really know what they're doing, but I love it casually.
Waku Waku 7: This game's mechanics are honestly borderline kusoge, you can't even cancel normals into specials. But I love the design and atmosphere so much. Tesse is really fun to play even in spite of the system mechanics.
NES:
Fire 'n Ice: A very rad little puzzle game.
Mike Tyson's Punch-Out!!: Just an absolute blast. I won't bother listing them seperately but also check out Super and Wii. Super's kinda the black sheep of the series, but it's still a good game, just not as good. Wii is an absolutely top-notch successor and I'm sad it didn't get any more sequels after that. The two arcade predecessors are honestly forgettable.
SNES:
Chrono Trigger: I am hesitant to recommend most JRPGs from this era if you did not grow up on them, because many of them haven't aged so gracefully. Chrono Trigger is the exception, this game is a fine wine. You may want to check out one of the rereleases though, or at least a retranslation patch, because the original translation was made on a rushed deadline and bound by heavy technical limitations.
Earthbound: A bit more of a slow burn in comparison to CT, but this game is carried by incredible writing. It's also required reading before playing Mother 3 next. You can skip Mother 1 though.
Kirby Super Star: Definitely the peak of the series, giving every copy power an entire moveset is a blast. Has an updated rerelease on DS with added extras, I do highly recommend this version, but DS can be awkward to emulate so SNES is fine.
Panel de Pon: Gamecube version is best, but if you can't run Gamecube then Super Famicom is good too. GBC is also worth checking out, in order to adapt it to the small screen the story mode has health bars instead of true CPU opponents, which makes it play rather differently.
Wario's Woods: The NES version is more well known since it was the system's last first-party title, and for whatever reason it's the only version Nintendo ever rereleases. But the SNES version is a notable upgrade, biggest thing it has is AI to play versus mode against. Versus mode is wild as hell, so if you've never seen it please check out the SNES version.
GBC:
Game & Watch Gallery 2: Holds a special place in my heart as the first game I ever owned. Has the best lineup out of all the collections, with 3 and 4 you can kinda tell they had used up all the heavy hitters.
Mario Tennis: An incredible tennis RPG. And Mario doesn't even show up until the postgame as a bonus boss, which I find hilarious. Has connectivity with the N64 version if you can get that running, lets you transfer your RPG mode character and unlock more content on both titles.