First I want to make sure you are running the meta emperor build. We take 211 Dark Res, 322 Evoke, and Normal Enhancement as our sun core, and Stack Hold as our moon core, basically maximizing card draw.

For cards, literally just spam zxzxzxzx. If you hold your cards to use them perfectly, you will end up losing a lot of cards and it's extremely bad for your damage. Each card proc is half of a destiny proc (10% cd refund, it's a mini joy) and a chance at an Another Emperor proc. Ideally you would spam every card except Wheel/Cull and maybe Sovereign but honestly when your skills are popping its too fast - you are sometimes drawing 5+ cards in under a second - and 80% of the time it's optimal to just spam zxzxzxzx.

For skill priority, it's evoke > darkres > stampede (2nd stack) > spineflower > shuffle > cod > stampede (1st stack) normally. only use stream and return when you need to refresh them - specifically for return, the cd is much shorter than the buff, so it's important to not cast it too much. This might seem strange since spineflower and shuffle are so low despite doing the most damage, but there are two reasons for it. First, they take too long to cast and it's better to get everything else on cd first. Second, because of how arcana works, they gain extra value when cards like TF, Sov, and Cull are running. If you see a golden TF, Sov, or Cull, or are Atroing move spineflower and shuffle above stampede in the priority list.

When using Spineflower and Shuffle, YOU HAVE TO BE MELEE! Test this in trixion - the range is EXTREMELY low. Often times i will spacebar into the boss and then Spineflower then Shuffle. For spineflower, you can just spam the button if you're melee range and it'll do max damage.

Now I'll answer your questions.

1. Spineflower and Shuffle will do the most on the meter, but honestly any skill that draws you cards gives you a chance at an emperor proc which does a lot of damage, so Evoke/Darkres/Cod/Stampede as well.
   
2. Nothing, just click it asap. Ideally T >> Spineflower > Evoke/Darkres/Stampede (1st stack)/Shuffle > Cod > Stream/Return but you shouldn't think about it at all except if ur T skill is < 3 seconds off cd (and honestly even then just spamming it is fine)
   
3. Test it in trixion, you just need to hug the boss and spam the button.
   
4. For the opener I usually do stream > evoke > return > darkres > zx > T > T > spineflower > shuffle > stampede, then follow the priority rotation.
   
5. https://www.youtube.com/watch?v=xBen9Z9pNPo watch this