I'll try my best to answer these:

1) Is it possible to add an Amex to WeChat as a foreigner? If so, would the WeChat Pay work at vendors throughout China? I don't think WeChat has an agreement with Amex so it probably won't work. Currently foreign Visa and Mastercard accounts still don't work either.
2) I do not see a Union Pay insignia on my Visa or Amex. Nope, it's a Chinese thing (plus maybe Singapore, I can't remember)
3) Would it be acceptable to ask the hotel to load $$$ on to my WeChat account? I would of course give them $$$ in cash. No, becuse you need a Chinese bank account and mainland ID just to get WeChat pay to work.
4) Is cash just not accepted? I've been in and out of China for 18 months, I still use cash everywhere since WeChat doesn't work for me.
5) Typically when traveling I just use a local ATM to obtain local currency, but since my cards do not have a Union Pay logo will my debit card not work at an ATM? A lot of ATM's (Bank of China, China Construction, IBCB (??)) accept foreign cards. Look for the Visa / Plus / Cirrus signs. If they have those your foreign card should work. Not all banks, or even branches, will work. Just remember which one does and you will be fine. There are banks with ATM's everywhere.