Kinda surprised at the vitriol James has for the story of Cindered Shadows and Three Houses. I'm not sure if you're just mashing through dialogue, but there's a reasonable explanation for almost everything you said makes no sense or is "dumb".

Balthus is the closest friend of Holst, who is seen as one of the best generals in Fodlan even by people outside of The Alliance, and has also knwon Hilda for years as a result. So it's not like they just up and trusted this dude with a bounty on his head for no real reason (and even his bounties are for a comically idiotic reason, which they're up front about; it's not like he's been out there committing war crimes).

Same goes for Yuri and Constance; Ashe is acquainted with Yuri through his being the adoptive son of a well regarded Kingdom noble, and Edelgard recognizes Constance as the daughter of former Empire nobility. So, outside of Hapi, the Ashen Wolves aren't complete strangers. On top of all that, the biggest driving force for the group from Garreg Mach to help them in the beginning is because there are lots of other people in Abyss who are innocent and need protecting. It's not like they decided to help these four people fulfill their life goals specifically at the outset.

Even the part about the passage to Abyss being some kind of unforgivable stupidity is a pretty big reach. It's not like Garreg Mach is some shanty on a side road.

https://vignette.wikia.nocookie.net/fireemblem/images/b/b8/Garreg_Mach_Monastery_day_concept.png/revision/latest?cb=20190704100726

It's a gigantic castle town built by dragon people and their fellow mystical allies in the middle of a mountain range. So yeah, hidden passages that the class hasn't discovered before or an entire network of tunnels and facilities underground doesn't seem that unbelievable.

I think Puyo Puyo is technically a spinoff from some serious JRPG which is where those characters are coming from.