Let me preface this by saying no country deserves to be glassed. Real or fictional. There are always people trying to do the right thing in the face of whatever ills plague a nation, and there are always innocents that will be caught in the crossfire. Wanton slaughter solves nothing, and you lose any moral high ground you may have if it comes to that. (Even if circumstances are exceedingly extreme and you have no other option... you still did bad. Necessary bad, but bad all the same.)

The "Lore Book says..." clause regarding the Garleans' mistreatment before magitek isn't necessary any more. Garlean Hojo Aulus mal Asina provides this exposition in-game while he and Zenos begin to explain to Fordola the SOLDIER program Resonant procedure. (I'm pretty sure they explained more than the cutscene showed. Maybe. Possibly.) Regardless, as with Nidhogg and Yotsuyu, it doesn't even come close to justifying the things they've done (and continue to do). It explains it, sure, but when you're attacking a generation of people who've done you no wrong you lose any moral high ground or sympathy. This paints their actions as vengeance, and the only way out of that cycle is to let bygones be bygones.

The Garleans aren't enemies of Hydaelyn... yet. If Varis continues to work with Elidibus that could very easily change. I hope to high Heaven it doesn't, but one should prepare for the worst.

I'm not keen on speculating how we'll ally with Garlemald. I agree it's more likely to happen than open war with them, despite current tensions - but without someone like van Hydrus or van Baelsar, a Garlean patriot with a sense of decency and honor, I can't see how.

Side note: despite past transgressions, Nero is starting to shift to the side of good, even if he's still insufferably smug and selfish about it. At least I think he is, but that could just be confirmation bias since I make no effort to hide the fact I want him to become a heroic instance of a Garlean.