While I agree Garlemald should be more fleshed out (or as fleshed out as every other nation on Hydaelyn), that doesn't necessarily mean their philosophy and goals are morally vague. They can have a long, detailed history and goals and all that and still be capital E evil.
If I recall correctly during the same interview he pointed out how monstrous and unsympathetic Zenos would be, Yoshi-P also stated that people weren't supposed to like or sympathize with the Garleans, and that Zenos' monstrosity was in part a response to that. People will always find things to like about the villains, but they generally acknowledge when the villain(s) are meant to be evil, not try and force moral ambiguity into a place it doesn't fit so the villain(s) can be sympathetic. Go in with a clear mind and see what is, not what you want to see, is what I'm sayin'. (Note: I do not hang out on social media sites, so people liking Charibert and Grinnaux perplexes me. The former is an annoying pyromaniac and the latter is a bloodthirsty loose cannon, both so smug I just wanna punch them in their smug prick faces.)
I'm still convinced Fordola is going to be Lyse's rival; beyond that, I'm not keen on guessing what she's going to do. While he died a hero's death, van Hydrus was the legatus of an Imperial Legion. Only the gods know how many people were mistreated and how badly under his command and at his hands; it's up to individual discretion whether or not that one act of sacrifice is enough to atone for the things he did. (The story frames that it was.)
Peace will eventually be made with the Empire somehow. I also agree that Eorzeans should not be so stuck up about embracing magitek, but that is their decision to make.
It's widely acknowledged that if the Empire got serious about conquering Eorzea, we wouldn't stand a chance. The Alliance could barely hold off the tattered remnants of the VIIth Legion at the Battle of Cartenau; even with Ishgard's considerable military prowess added into the mix, there is no way Eorzea could stand up to the full might of the Garlean Empire.