1. Very very little chance. There was a time in the distant past when there was a deal that included dining plan. There was also a set of YC deals a couple of years ago that had "free dining" listed and I suspected might have been CL - but when I pushed the question with Priceline, it turned out it was an error on their part. (They make mistakes with amenities all the time.)
2. On PE they only guarantee room for 2. Because the vast majority of Disney resort rooms cover 4, that's not really an issue. But, IMHO it is much too risky booking a non-refundable deal and hope you get a room that fits 5. On Hotwire, you can add people to your hidden deal search, but they WILL increase the price, sometimes a lot. Still wouldn't book 5 on a hidden deal there - and the better method on HW is too book with 2 and call Disney Wholesale and/or HW to add kids after.
3. Hard to say - both sites sometimes drop deals all the way out to the end of the year. Lately PE is mostly a dead zone, and after a single large drop in the Spring, HW seems to be rolling things out in the 4-8 week time frame.
4. They are not actually left over inventory as I understand it - but blocks of rooms resellers made a deal to sell, and I suppose are trying to unload on the cheap to avoid a loss. (Some, of course, might be direct Disney offerings too.) You'll often find that when Disney doesn't have rooms for a certain resort, there will still be some available on the resellers.
Over the last few years there have been many blocks of deals for high-attendance dates like holidays and school breaks. But, no real patterns I've been able to discern - and believe, me, I've tried.