What is the cost, and what does it cover?

To leverage the equity for financing my next deal, a second mortgage seems like a more cost-effective option.

They get a higher return since they are covering housing cost, have the $250k per spouse primary residence deduction in gains, is a savings account, higher leverage position, lower interest rate possible, will pay more premium for location, etc.House might be overpriced for REITs but not for home owners.  

So about 8 houses per year will do ya....but don't forget your costs of doing business AKA overhead....Very good point!

Kit showers or tiling, or tiling the shower (hot mop or tub shower then tile the rest) all are different3) what is the cost to renew the floor with a LVP?

You will likely need to rehab for any strategy but Co-Living will be more bedroom additions which is less cost compared to kitchen and bath remodels that are more needed for furnished rentals. 

We got started, but they hired an appraiser and charged me $750, while typically appraisal costs around $500.

I was a student they offered to sponsor a deal for and it has cost me everything, literally. 

No but keep track of those repairs as it will add to your cost basis when you go to sell it and have to pay capital gains.

Imagine if it cost 1000 to send out 50 letters but half of them responded and half of those wanted an offer and half of those accepted an offer and half of those closed and half of those resold for a net 50k.