How do you get Richard of Gloucester to avoid usurping the throne from his infant nephew, and instead be content with merely being Lord Protector for a period? And how do things play out in such a scenario?
If you want a Lord Protector period, then maybe a compromise between OTL and the one suggested by vandevere?
If Edward IV dies in 1485 or so, then Edward V will likely be seen as more his own man, and less as the dowager Queen's son. The problem is that once Edward IV dies, there's precious few powerful players on the board to stop him making such a move.
If Edward IV has to die on schedule, how about this? I suppose if Tudor, or another figure, stirs up a rebellion in 1483, and the Lord Protector has to stamp it out, he could be distracted for a while. The longer it takes to stamp out this rising, the better chance Edward V has. If Richard has to spend the summer retaking castles in Northumberland - Bamburgh and the like were forever being declared for Lancaster - then by the time he feels able to move South, it may be too late for his usurpation. The Woodvilles can't hope to sideline the loyal uncle who's been fighting to keep his nephew on the throne, but he may have more problems taking it as his own.
BG, last time you popped into a War of the Roses thread, you mentioned you were reading a book by Hicks. It may interest you to know that I'm currently receiving lectures from him once a week at Winchester, and one of the Case Studies we can look at in November is Richard III, led by the self-same Hicks.