Check to see if the township considered the commercial use an existing non-conforming use in a residential zone.

I have a potential tenant that has listed a treatment center as a former living situation (he was there for 18months prior to the existing 18 months somewhere else). 

Simply ask the sponsor if they would be open to connecting you to their existing investors who've known and invested with them for a long time.

I am seeing some water seeping into garage under the balcony so I would like to make sure to tightly seal the gap,I was thinking about removing worn out sealant and pour there some liquid rubber and then on top of the existing tiles I would like to install LVT to fully water proof as well as enhance the looks, is it a bad idea to have LVT outdoors?

Rents have increased and cashflow deals still exist!

Some hybrid models exist, @Brooke Roeder mentioned Mynd and I have had several client use that with good success.  

rezoning is not feasible, it's very costly. if it's zoning a 2 unit then you are in a so called pre-existing non-conforming, check the floor area ratio if you are already over then there is not much you can do, however if you your FAR is below then there are potential opportunities for the garden unit considering the newly passed ordinance. 

====================So what I see is that as long as the occupancy of the FHA-financed property is primary, the existence of other investment properties should not be a deal-breaker.

If my goal is to rehab up to market standards and therefore market rent; how or is it possible to not kick out existing tenants as l am taking over as the new owner of a multifamily property?

So, I can purchase another home, add sq footage to the existing rental property and use that as a write off/increase rental potential, or not do anything at all.