I realize it's a sellers market but the markups I'm seeing seem excessive.

If I recall correctly, dry funded double closes have been misidentified as mortgage fraud in the past thus leading to excessive concern.

At what point do you think I should I recognize this as excessive maintenance costs?

What is the appropriate amount of Umbrella coverage does one need to ensure adequate/robust (but not excessive) coverage? 

Use that money to to buy about 1.2-1.4MM in investment properties whether it be 3-4 SFH or a MFH (not familiar with the area), and use excess cash for some value add cosmetic work if available.

Excess traffic, the neighbors will complain about all the cars2.

What I choose to do with my "excess time" is give it back to others who are pursuing their own journey to financial independence.

And for heavy real estate investors, an important thing to note is the excess business losses.

I worry about the liability and also booking stays in excess of 28 days. 

You don't have excess capital to put into real estate deals.