There is no way for the two of you to acquire the SMLLC because it would be a two (2) member LLC and not a SMLLC or disregarded entity. 

Acquired just 2 1/2 years ago), or save keep building up the cash flow to use for future expenses and other aspects of the business?

We have a fixed purchase price so it is completely up to the seller to acquire the remaining units at a reasonable price and consolidate the property on their dime.

What advice would you give to help a first time house hacker in acquiring their first property in a hot market?

The fact that one closing can result in acquiring multiple doors vs one, and realizing that if my SFR is vacant that property isn’t doing anything.

If you have access to the MLS this data can be acquired by searching for sold and expired properties in your area over your desired time range.

There is no limit as to the total (aggregate) number or value of identified like-kind replacement properties permitted under the 95% exception as long as you actually acquire and close on 95% of the value identified. 

We have quickly acquired about 75 units in western MA.  

-Building permit can be acquired for only permit fees, structural engineering, soils testing and surveying fees.

Also, how did you remedy the situation?