Most mortgages aren't.Without paying off the mortgage it would be a sub-to deal and I think risky for everyone involved

The OP is currently exploring conventional rehab loans (presumably 203k or similar) which would indicate that he is intending to owner occupy, and therefore hard money loans are likely off the table (as most HMLs are not NMLS licensed and don't make owner occupant loans).That said, to the OP, if you're looking at a conventional rehab loan you're just going to have to accept higher costs, less flexibility, and more red tape.

Then unclaimed property will be Auctioned off by The State.

You need to have consistent inventory and be a serious player to pull #1 off, assuming you are in a competitive market.

If your income is strong and taxes are accurately reported, you have a shot. 90% of investors write off more than they make so this upside down calculation is what causes the average person to qualify for HELOCs.

There are only three instances where it makes sense to buy solar panels for your house.1 - You are living off grid. 2 - You live in a state that has sky high electric costs AND great tax insensitive.3 - You have lots of money, don't care if you are paying more, and want to feel like you are saving the planet (even though you really are not). 

Also with the right insurance you can take guests off Airbnb if they'd like to extend their stay or come back in the future. 

If I did not report expense for my 2023 tax, how would I write off my huge expense (~20K) for my future profit then?

It takes less than 30 seconds to take the arm off and the glass/junk filter out of the dishwasher.

I believe the most common path is to use cash, private, or hard money, and then to refi into something traditional once the work is done.Sorry I was 1% off.