In response to question number 1 the ink system is fairly simple. In a recent newsfeed post Tapas' main site they explained the many ways to earn ink which can be read here. As for how ink translates to currency I think it was roughly 1,000 ink drops to the dollar (USD as far as I'm aware) but I've also heard that ink values can vary depending on whether they were bought with money or earned through one of the free ink options. When you give ink to creators there's the portion that goes to them as well as Tapas' relatively small deductions that goes to them and the app hosts (Google and Apple). Once a creator earns $25 in total revenue they can transfer it to their PayPal and that's cash money in their pockets.
As for question 2 i think just about everyone has a chance for being promoted whether new to the site or veteran user and with any number of subscribers. In terms of curation as far as I'm aware its specifically for the New & Noteworthy section as well as the collections on both the main site and the app. Popular and Trending are based on reader activity on any given comic or novel as it mentions (i think) on the main site which is likes, views and subs within a span of time.
As for the handpicked stuff I know fanart is a no-go along with any...meme stuff. When it comes to the rest of the site content I've noticed it usually comes down to clarity/readability of the art and text, mobile friendly, not just in format, but the content is something a person wouldn't have to strain to read on their phones or other mobile devices. Art that's eye catching or of a specific level of quality too (but not based on biases) there's obviously the trends in art that's in premium comics but really digging through some of the new and noteworthy sections and certain other staff picked collections you can see an array of styles, genres, and formats sometimes too. Both black and white and color comics have equal opportunity just like various types of novels but clarity for the most part seems to be the key factor.
Question number three is the only one I'm not too sure of since all deliberations will be through you and the artists or writers or whomever you choose to work with when it comes to legal terms it'll just be up to what you communicate. I know that collab posts are an okay here on the forums but I it's not something I've read deeply into. I know of some folks who have hired artists or worked with artists on long term projects but I can't give any insight on the ins and outs.
I hope what I've given makes sense and is helpful in some way and apologies for anything that seems to lack clarity but I wish you the best in your time here on Tapas 
