A friend suggested the use of a vaporizer could be the culprit (one of their children is asthmatic) and a windows company who went to inspect it said they had covered the weep holes, which would definitely cause mold.

Hello All, I am not sure if there is a rule of thumb to how much a design should cost but I received an estimate of $23K for Civil engineering work (site plan, storm drainage plan, site grading, soil erosion & utility plan), $25k for  Architectural and structural and $11K for MEP for 12 units townhouse (2 bed and 2.5 bath) on a slab. 

You just really don't want to take this on if you can avoid it because you will automatically be in the hole with not only a non-paying tenant but the out of pocket costs of the eviction, which can be substantial.

In this market where asset is having high valuation in relative to rent income and money is expensive, if Becca is buying another OOS she would create more infinite rabbit hole as upside is quite limited and may need more time to work out.

Right on a small chain lake, 18 hole golf course, cool clubhouse etc.

I would just put a few more snap traps around, seal the holes to the exterior that the mice are using to get in and out (sprayfoam is your friend for this), and make sure the tenants remove any food or trash, water, whatever is attracting them.

Is there holes in this strategy?

YES, the unspoken TRUTH of it all is that a MLS listing STARTED life as an OFF-MARKET deal that a REALTOR dug-up, discovered, ran down the rabbit hole of analysis, confirmation, negotiation, and MADE an ON-MARKET purchase opportunity. 

And I note you completely skip the mountain sized hole I blew straight thru your premise.

Otherwise, I have all the time in the world to watch you keep digging a bigger and bigger hole for yourself.