Thank you, I hope you love it!
On your first question: Net-net investing, particularly in the U.S., HAS gotten much tougher. There are objectively fewer of them as the economy has shifted from being dependent on property, plane, and equipment to tangible assets being a minority of those for the companies in the S&P 500. With that said, there are still these types of opportunities on the fringes (and more internationally!). You only need 1 or 2 a year to make your returns. Can Buffett find that one or two? Yeah, I think so. It's possible it would be international than here, and my guess is it may be in even smaller companies.
I'd add, Buffett also did a lot of arbitrage, and he would also use this. I think he would pivot to companies more dependent on the earnings a little sooner than he did if he were running today, especially as he scaled, but he would still do a lot of balance sheet-oriented investing / activism.
On the second question, I think he can definitely get 50% on $1 million deploying broadly the same tactics he did in the pre-partnership years and the first half of the partnership (he did fewer net-nets in the second half of the partnership, such as Amex and Disney). I think the anchor scale presents is pretty drastic, though. Meaning the returns on $10 million are way worse, $100 million are way worse than that, etc.
I think there are a lot more investors that had outstanding returns for 10+ years than is commonly known. But they sort of stop investing other people's capital after accumulating their initial fortune.