1. Who decides what recording studio and where?
That would be you in conjuction with sellabands music manager allocated to you. Together you pick the right studio for you. They have a worldwide database of 'sellaband friendly studio's' , but that's not a must. They will prevent you from recording in your friends basement if it has no known trackrecord or can't produce professional results. In other words, Sellaband might be more concerned than you about a professional endproduct.
2. Who gets to pick these "professionals" to assist with the album?
That would be again you and their musicmanager. If you come up with a good producer/professional and Sellaband is ok with that, all is fine. If not, they will assist you in getting a real pro. And believe me, I have met some of them (for example Tony Platt who is known from Paul McCartney, AC/DC, Bob Marley etc.) and believe me, they are well up to the job and fully back up the Sellaband concept. So they don't do it to get a fast paycheck, but for the love of the concept.
3. What's the production quality like?
I think from reading the above you can say the quality is just fine

. But in the end it's all determined by your professionalism. You can't make a jewel out of crap. It will be shiny, but it still will be crap.
4. Are they talking net sales, or what, with regard to the 50% figure?
This I don't know exactly. I do not work for this company, I just know somebody who does. So i have been talking a lot about the concept with this person, but do not know all the 'deal' ins and outs. But it's all on their website. All I know, the goal of this company is not to make money, but to give the artist a chance. The owners are big players in the musicindustry, who quitted topjobs to get this concept working.