I will try to help with some of your questions

:
1. A one bedroom sleeps a max of 4, and should be fine for two people (that's what DH and I have booked for our couples trip in May). You can check out the various layouts of villas by section here:
https://www.starwoodvacationnetwork.com/villa-collection/sheraton-vistana-resort/villas#floorplan-1 . There are two types of 1 bedroom villas.....one has a balcony (and substantially more square footage), the other does not.
2. A one bedroom Sunday check-in will be in either Lakes or Cascades. Only Lakes and Cascades offer 1 bedroom villas (Fountains are all 2 bedroom configurations). You can check out this map for which buildings you might be assigned to: Sunday check-in buildings are in green.
https://www.mystarcentral.com/CMS/en_US/svo_media/CMSMedia/PDF/resort_maps/svr_resort_map.pdf
3. Yes, you can. Use this form about 2 weeks before your arrival:
https://www.mystarcentral.com/services/diamond.jsp?propertyId=01
4. No parking or resort fees at SVR. You will get one mid-week tidy as part of your timeshare stay (on Wednesday, I think, for Sunday check-ins). The villa will get a full cleaning before your arrival and a full cleaning again after you check out and before the next guest. You can arrange additional cleanings, for a fee, by contacting Guest Services.