He got some kind of grant for extending it another 10 years.She recommended I put it an offer for $150k, and he agreed to it.

Look at making it an extended stay motel .

If you have a valid lease in place (and in your scenario, I would never do a formal lease/rental agreement...you should treat this like an extended stay hotel, which doesn't give the tenant an equitable interest), you should follow Sec. 92 of the Texas Property Code.

There is no locked rate period like there is with conventional. 

But our offers aren't sight unseen, we will always have an inspection period in our offer.

If I were you I would be willing to honor the lease amount and this person as your tenant for the time period they are prepaying for. 

To answer your question- you should be able to lock now but in many cases you’ll have to pay a % of the loan amount to extend that lock all the way until April 2022, just depends on your lender.

Since you are likely assuming a contract from the wholesaler, said contract should have an inspection period.

Another thing the potential new banker mentioned (he’s a friend of a friend) is that the current bank might be able to do an “allonge” to extend the current loan with another 7 year rate lock. 

Number of claims over a period of time can create problems for getting insurance or can increase your rate.