Additionally, holdover tenant is now subletting out the rooms making income and advertising it on 168.com.

I personally am okay with having a cosigner for a tenant because this adds an additional person who is financially responsible for the lease if the tenant is unable to pay rent or for damages they cause. 

heloc terms are not the same as fannie mortgage terms (like @Chris Mason mentioned)for instance I have a 90% heloc on an old primary that was ZERO cost, and the rate is prime -.001%. this additional LTV might help your cause. a heloc is also not necessarily due once you move.

In addition, in our lease agreements we require tenants toobtain renters insurance for theirs and our protection. 

And that disparity only gets wider and wider as you add more properties as most channel managers discount very heavily once you have a lot of properties whereas Hostaway's discount is very minor.30 properties on Ownerrez is $250/mo compared to $750/mo on Hostaway.

for such a small addition, skip the architect and just get a general contractor

Common additional apps are things like Receipt Bank or Hubdoc for receipts.

These forms provide a written confirmation of the terms of the oral rental agreement and help prevent misunderstandings or disputes down the line.By having tenants sign estoppel forms, you can verify important details such as rental amounts, deposit amounts, and any additional agreements or responsibilities.

Additionally, basing the loan amount on past below-market rents seems unfair, as lenders should consider current market value and potential rental income.

There done, this is an extinction level event for discount/flat-fee brokers because there discounted system was reliant on an ocean of ready buyers/buyer agents bringing buyers because that's literally how they could be discount, they just listed on MLS, did the bare minimum.