They said that we could get grandfathered into the program and offered the same terms to us.

It’s critical you know your docs and are familiar with your terms in those documents.

they are good about responding to emails, but may take 6-24hrs and many times they don't answer the question fully so you have to go back & forth a couple of times which is frustrating when you have to wait long times for responses.

You may need permits for plumbing, electrical, and structural changes.A HELOC can be a flexible and cost-effective way to access funds for a down payment on another property. 

@Christopher Lynch  NH has some amazing area's for Short Term Rentals.

Can a 25 foot long 6"x4"x.210" 6061 aluminum association I-beam can span a distance of 24 feet?

Sale-Leasebacks have inherent challenges because the tenant/seller is incentivized to market the property in such a way they get they price they want, but if the entity who signs the lease is weak and there's no personal guarantor, they could stop paying rent the day after you close leaving you high and dry with very little legal remedy.I don't know your market, but in my eyes, any 10% cap offering is either an extremely weak tenant OR a tenant that has very little term remaining with no plans to renew.If I were, you I would very seriously consider how much I like the real estate and what my odds would be of refilling the real estate with the same or better rent and/or a better credit tenant once this tenant goes belly up.

So....... with my previous CPA (who has since unexpectedly retired) we did a cost segregation study on an asset that was placed into service in 2021 which broke down the bonus depreciable items into 3 buckets, a 15 year bucket, a 7 year bucket, a 5 year bucket and the rest would be on a 39 year straight line depreciation schedule as it is a short term rental.

It would fix your cash flow problem, but that comes with the furnishing expenses and a little more work than long term rent. 

You are NOT going to change their behavior.Charge the late fees, see if you can also charge a Daily Late Fee to discourage them waiting too long to pay.When is the lease up?