I have stayed in the Hainan Hilton at PEK many times, but I don't know much about the deal HNair expats have if they want to stay there. Prepaying or not I don't know. I did meet one expat that was actually living there. Our "deal" is that we can stay at company hotels for free if we fly that day. I have never asked any of the HN pilots about this. Sorry. I believe it costs about 170 rmb (27USD) on a daily basis.
I know that Chinese pilots at my airline don't get paid for DH, and they can do it on their day off, and the company counts it as a day off.
The rest of your questions cannot be answered honestly, by me or the agency trying to recruit you. Nobody has done it, and the details have not been worked out. And after the details have been worked out, it will change. Numerous times.
The contract agency will tell you what you want to hear, at the end of the day they are sales people.
If you are based in Seattle, some Captain will have to deadhead, either to or from Seattle. It will only be Chinese DH-ing if it benefits them. Right now the flight requency to Seattle is seasonal. Winter time 2-3 times a week, summer maybe 4 days. I asked the FA's and sometimes they spend 4 days in SEA. Usually 2.