And those are old pre-pandemic numbers. The amount the Fed holds has gone up a lot (aka quantitative easing). So the number is even closer to half now.
Fed debt is effectively monetized or will be sold back to the public later. It doesn't change anything. At best you can make a hand waving claim about it meaning we can get away with a bit lower interest rates while monetizing.
The rest held by the gov is mostly social security. It represents an obligation to make payments to people outside the government (under current law). Not sure why anyone would think this doesn't matter.
We'll learn soon enough whether the US defaults, monetizes, or substantially reduces SS payments first. Suppose tax increases are theoretically possible, but they would need to be substantial to cover the deficits of 25% we run regularly now.
Nope[1]. Local maxima at 42.5% circa 21Q4, but currently 38.3% and trending down since.
[1] https://fred.stlouisfed.org/graph/?g=18ZDm