I am also thinking of using a skip tracing service to see if I can get additional contact information.  

Is there any additional fee that would be charged to compensate the individual for their time?

Given the additional thickness required, you may also need some additional structural framing -- different can of worms, but this is often very doable.  

The ceiling joists will not be strong enough to put additional weight on.

Or do I have a legal right to un-alter the premises?

So my total cash in, after I put this additional $3k, will be $51,5K and expecting to flow about $1,000 per month. 

I've taken my first property (8 unit) and not put a single additional penny in and used appreciation, smart buying/selling to turn that to 1200 units in 10 years without any outside investor $  (okay, 11).   

I also find investing in notes in other markets with slightly higher returns provides some nice additional cash flow without having to deal with any issues of managing tenants. 

I expect to be able to get an additional $200/month more for rent when it is listed, although we are considering using it as an AirBNB instead, as we have had a wonderful experience with our other short-term rental. 

The assessor looks at comparable sales to come up with the value of your property, they don't actually look at what's been done to it unless there is an addition.