Yes, the last 2 years have not been typical, so I wouldn't bet on those prices. 4% is a more typical value for Texas based on my experience, but here is some data as well: https://www.bizjournals.com/houston/news/2018/10/17/how-houston-homes-have-appreciated-since-2000.html

Yes, I did account for the pay down of the loan and broke out that number in the story (appreciation plus equity paydown).

Again, 3% is the typical rent increase. Actually, the calculation for 20% IRR on rentals assumes you break even, so rent only increases as much as expenses. So it is the same on both sides, not much higher. You can look at the article I linked on leveraged appreciation to understand where the 20% comes from.

Only principal and interest stay flat, all other costs increase with time including taxes (based on appreciation), insurance, maintenance, utilities, etc.