(a) Retro gaming is an extremely wide at this point. It houses multiple generations if extreme upgrades from PS2 to Spectrum. The further back you go the more restricted it becomes. So if you want to get into retro, I think it might be good to move backwards to gain a tolerance. As you go back QoL will slowly get worse as we've built upon them over the years.
(b) Gateway games are difficult because for me a gateway is a way to gain interest, it was never just picking the best of the old games. When Megaman 9 came out it was good enough that I went back and started playing the old games. Because I enjoyed it and it emulated the originals. If there are some retro-inspired games you enjoy, going back to the games that inspired it is a good gateway (for me anyway)
(c) I'm not sure how this feed feels about it but feel free to use save states. Arcade difficulty is real and sometimes saving can be complicated. Just remember that they exist when considering an opinion. Your feelings on contra may vary depending on if you use save states to beat it.
So if N64 is your "modern" retro I'd probably recommend Mario 64, Ocarina of Time (slow start be aware), Kirby 64 and Paper Mario. From there if you like Mario 64 and Want another Banjo.
I think SNES JRPG is a bit tricky. I don't really remember my avenue into it. I believe it was GBA and DS JRPG that gave me a tolerance for more simple combat systems.