4) Apart from update patches for classes, 4* challenges for Kingdom story maps, more story maps, skill AW'ing, rainbow crystals, more TP units, revivals, weekend dailies, and some new character art (example being Anna), I don't think there's much difference between the two versions in terms of gameplay and UI looks. Though DMM Aigis will change its UI in the near future (around this July/August I think).
5) I use video guides on YouTube to deal with the DMM event missions. Look up Youtubers atarimesan, sennen war, SLOWLIFE, and yuki hizuyama for good common unit guides.

I agree with Asdf1, DMM was certainly hard at first, but got somewhat easier and more enjoyable as you gain levels and some cool event and premium units. Hope a Miruno revival comes up soon for you, because she is probably the most useful event unit yet. I mean she has the second greatest range after Aisha's broken skill awaken range. This is just my opinion btw. I don't know any other units who have monstrous range as those two.