But they have been fired and have residue on the inside.

After-the-fact I found most areas had little adhesive/mortar, and the living room had a mix of three different types of mortar. 

If you are looking at a $15k renovation, keeping them where they are for the next year while you build your residual may not be that bad. 

If you buy used, you need to check for hazardous chemical residue in the unit, if you want to use for housing. 

Got that jetted and cleared, but now have a messy residue left behind.

The problem is not with the flooring but the subfloor  Since the home is 100+/- years old the only sure way to solve the squeaking problem is to remove the flooring and replace the subfloor, ensuring it is properly attached to the floor joists with construction adhesive and nails or screws.I would also NOT recommend putting new flooring over the top of this existing flooring as that is not correcting the original problem but adding to the original problem.

Work backwards and do some residual land value on the entirety of the project.

The potential residual impact is TBD.

We have done thousands of dollars worth of work to mitigate smoke smell/residue in a house but it wasn't our rental at the time.

So far so good and we’re buying an ozone generator to take care of the residual smell which is minimal at this point.