These are very valid and scary questions. I'm sure our uber-experts will chime in, but until then here is my take.
1) The
DVC Owner can cancel you reservation at anytime prior to your arrival. That is the risk you take, and why the rates are so reduced compared to Disney.com. But know these facts. If the reservation is cancelled more than 31 days before your arrival, the member is free to do pretty much as he/she wants, as the points are returned to their current use year. Between 1 & 30 days, the points go into a holding account, which really diminsihes the 'value' of these points along with their usability. At the time of arrival, if ressie is cancelled or shortened the affected points are lost. This is why most renters have a 60 day before vacation final payment policy.
2) You can check you reservation once it has been changed to your name and you have the confirmation # by calling Member Services (I can't post the 800 # here). You may make or modify your own DME and DVC-DP options directly with MS.
3) If a deal seems to good to be true it probably is! However, hear on the DIS, if you rent from a User with hundreds or thousands of posts, if you ask for and contact prior renters of this User, and check out their eBay rating, you should do very well, and be very safe. And save >50% off Disney's Rack for exceptionally great digs!
Ask questions, read prior posts, get comfortable, and actually contact prior renters and you should do fine.
The only 'horror' stories that I have read here on the DIS are DVC owners who go burnt by their renters!
Hope this helps,
-Tony
ETA: As noted you can make your own DME directly and order the Dinning Plan which must be paid at time of arrival, and must be ordered or cancelled at least one day prior to arrival. Everyone in the room must purchase it for the entire length of stay!
Also I used to buy tickets over at mousesavers.com, they have links to discount sources (very reliable)