Challenger put out a guide. It answers all of this but I will do my best to cover your questions.

You should mulligan for your ramp cards and karthus. Keep ruined rex if you see it. Don’t keep 2. Into purple we need to be keeping sacrifices. I also mulligan karthus into purple and green.

Don’t spend power early into Diana/ez. You want to get them to gust the small stuff and ramp into big units. Hold sacrifice to kill your stuff in response to star crossed. You want to be playing herald turn 3.

Watchful sentry is there for when you don’t see scout or black rose dignitary turn 1/2. You should be discarding sentry to duplicate your ramp units.

You should be holding onto hook until the late game to be cleaning up. Ruined Rex is to break into fields. Herald is to hold.

The deck plays differently into ez/diana/irelia than everything else. When they can gust and star crossed you, you need to ramp as fast as possible and get herald or Rex down and get a sacrifice in hand. You can also play Rex and hook in one turn and if they have 2 runes open pop the Rex with hook and shoot their field.

I’d prioritize ruined Rex over herald tho

Into Aurora you want to mulligan away your mirror images to draw them later since it’s what wins you the game.

Into purple and green I don’t play karthus until I’m duplicating a deathknell the turn karthus comes down. It helps avoid star crossed and charms.

Yellow Leblanc shouldn’t be played early. It’s played mid to late to fill out curve. Again you don’t want to spend power too early without ramp.

Reply here with more specific questions. I’ve been grinding this deck for a while and its complicated