Pricing tools that exist on the market are decent, but you also need to know how to control it, otherwise, it will go nuts with unrealistic rates far out and extreme last-minute discounts. 

As far as the sale of a property with a mortgage on it, that all gets sorted out at close. ie, you bring your money, the bank gets what they have coming, the realtor gets what they have coming (if there's one involved) and the seller gets the rest.

Would your STR business even exist without Airbnb?  

Talk to a mortgage broker or lender experienced with LLC-owned properties before transferring ownership.

Quote from @Dave Schimmels: What is a good way to get accurate rent comps with existing tenants based on the area?

This will still qualify for a residential mortgage and you have a better shot of completely offsetting your entire payment.

You're going to end up paying a lot more interest on a heloc than a 2nd mortgage, if you intend on holding that debt for more than a couple years. 

If you have more than one LLC, each LLC should have its own set of accounts.Checking: Collect all income here, then use it to pay bills, the mortgage, or maintenance.

It may be best to partner with someone that has done one before or somehow is a local expert on lot splits.If you can add a bathroom into existing space on high cost property it can add a lot of value.  

I'm an investor and also a licensed mortgage broker.