The tenant failed to mention that that was the time they were going to be filming a "home movie" in the water bed.

TLDR, it's like a 5 gallon paint bucket filled with money.

I believe that the shower in the second floor bathroom is causing water damage and creating the mold in the first floor bathroom ceiling, which may require removing the first floor bathroom ceiling. 

In that same 6 years I have had to replace the stove and water heater, have the dryer and oven fixed, and had a tree trimmed and an eve repaired.

Consider putting in a separate electrical meter, a separate water meter, and separate gas meter.

Also study costs of roof replacements, furnaces and water heaters.

Go into Home Depot and see what a gallon of paint sells for. 

She didn't think the case holds much water but it could be very time consuming resulting in high fees.

The water is always cold, but at least in the summer it is nice outside of the water and the winter air temps are probably higher than the Tahoe winter air temps. 

Without knowing more about the location though, if this cabin has direct water views and is able to sleep at least 6 people, a fully renovated version of your cabin could probably pull in at least $600/night in the summer without much effort.