1.Yes, unknown.
4. Yes, but you can only use 1 gift card per purchase and a max of 1,000 for the card. So, you would have to use a credit card for the leftover 63.94.
5.No
https://dvcexplorer.com/fin/m/Riviera_Member_Welcome.pdf
Current MB is 20 dollars per point and WH is $1,000.
So, a 250-point contract would be $43,000 plus closing costs and doc fee.
In order to book with 2 memberships, 1. you can transfer points from one to another and make a reservation or 2. book what you can with each membership and the have MS merge the two reservations.
Now I can be misunderstanding your statement about a different membership, but if you are wanting...
https://eventswithdisney.queue-it.net/?c=eventswithdisney&e=dvcmmepcotswid2023&q=a6771422-a4e1-45ad-8062-a2286a93f2f0
Link for anyone the needs it 537 left.