1. Soldier 1 and 2 is totally fine when they have a different style!
2. You can add previously desgined minifigs, but I think voters will appreciate new ones. And like you said, it's maybe tough, but that's also the fun challenging part!
And yes one entry and digital entries are allowed.
Probably somewhere during summer.
You could use for not showing the backside if it's not worth mentioning. Are find a way to cover it up, with headgear etc. But no photoshopping faces. And yes like last time, no 3rd party stuff.
in the second category are different mocs allowed, just like in the final Book Challenge.
I assume you're thinking of working together with soccerkid? For the second category you could collaborate, but it should be presented as one entry by one of you in one topic.