BTC owed 500,000
Interest per day BTC 5000
Generated BTC per day 6x24x50 = 7200
So he would only need to generate 69% of all Bitcoins to make good for the interest, without paying back anything of the debt.
No, he would only need to generate 51% of all blocks to be able to repay all his debt with 1BTC. 51% with asics is fairly trivial right now.
You keep making these claims. 51% attack doesn't allow you to spend coins which are not yours. So while in theory he could technically repay investors by stealing from them in a 51% attack you couldn't do it with 1 BTC. He would need 550K BTC today and that number is continually increasing by 4 BTC per minute.