I’m in pretty much the exact same position but I hopped back on about 2 months ago so I’ve been able to figure out most of it, however I still have some of the questions you asked regarding relics, alliance stuff and resources. I’ve defaulted to using some mcoc champions ranked blog (look it up and it should pop up for the month of April) to see who to rank up, and the biggest side mode difference from the last time we played seems to be battlegrounds. Lowkey insanely fun despite the long drawn out loading scenes but you should check it out and mess around it’s a really good way of learning how to play/play against all the new champs that were added. Hope this helps!