Interesting project indeed. Just as long as there is a link/proof that the charity has agreed to take the 5%, that the funds are of course given to them, and that the charity has a license to take the donation.
I commented recently on this type of game though, and how it is open to only large whales coming to win. I lost an amount I still feel painful about not too long ago playing exactly this type of game. They let you win a bit, then you just lose everything to chase the button.
Having the last person as a jackpot winner will effectively stall the game as people will be waiting for that sweet spot at the end of the given time. You can certainly make it more engaging by making few changes.
Instead of having one jackpot winner, make it 4 at random numbers like 15, 47, 80 and the last one. The numbers I have given for illustration, you can make it any random numbers and don't announce those numbers upfront. It will keep the game ongoing as participants will be trying to hit those hidden sweet spots.
Just a suggestion, think about it!
Great suggestion. Make more than 1 prize, and randomly, so people have incentive to click it without the timer ending. Cool idea.