I feel like this deal pencils at $365-375k but need to add a bathroom, redo kitchen and existing bathroom, close in garage port for more sqft, + cosmetics and new appliances.

If the lender won't budge you can always refinance the parcel containing the existing home and remove the developable parcel from the new loan request.

In larger mutlifamily properties I typically increase my existing expenses (non-mortgage expenses) by the same growth rate as my revenue.... assuming I have up to date and accurate existing expenses.

Hire a Architect to provide drawings with a lot of details such as existing house elevations and floor plan, proposed elevations and new floor plan, roof, wood deck, addition, etc.

You made the same mistake I did which was to agree to renovate an existing space that someone was living in.

It doesn't exist yet. 15% down will most likely be at a high 8% rate and you may get an LTV reduction with it being a short term rental. 

This policy will provide additional coverage above what your existing policy covers.

So for them this is the only way that currently exists in this market to get a managed, complete property with little or no work on their part.

Can I use a previously existing LLC?

Unless there exists some alternative revenue source for a Paperstac-like platform to profit from (beyond transactions) or some amazing value proposition not already provided by Paperstac, it's hard to envision a similar site for smaller players doing well.