I imagine they will use a sort of stratified sample, where they want 50% of the riders to be sub 6 hours, 10% to be 8 hours etc so they will put all the sub 6hrs into a hat and draw out 50% of the ballot, for example. Means its impossible to tell exactly what your chances are. You could be competing against 70,000 others for 12,000 places or against 5,000 for 3,000 places. This is just my guess of course! They would also want to be able to tell if you are likely to turn up so an occupation of airline pilot may mean you are unable to make the start. I would hope that all of us that turned up at the start this year would get some brownie points as they can be pretty certain that we will turn up whatever the weather next year as well!
My view on the charity second ballot thing is if you had one place between two people, one who had donated their fee no matter what and one who would only pay you if they got in the temptation might be to give it to the non donator, that way you get both fees!