Have had to send multiple quit notices and been on the brink of going down the summary ejectment route before payment finally came in at the 11th hour, so just trying to get ahead of this before the cycle inevitably repeats.While the process itself looks pretty straightforward, since I'm out of state I reached out to a local landlord-tenant attorney for assistance, but he told me that I would have to physically be present at court in the event that the tenant showed up and contested since the attorney  "would not have personal knowledge of those facts, and an affidavit from [me] would not be sufficient".

I think you are right that this is sufficient.

In my case, I have an investment property I want to purchase, but I don't have sufficient interest reserves for a hard money loan.  

I had a brand new AC unit installed in July.  Yesterday, the tenant put a repair ticket in, in it she said "ever since this new AC unit was installed, the house will not cool below 77 degrees."  I just got off the pho...

As you would be a new landlord, this would be more square footage to take care of and more tenants to vet and manage. 3-4 unit properties also need to pass the "self-sufficiency test" from a lending perspective.

I have sufficient space in the side yard and most of the pre-fab detached ADU options I've seen so far cap out at around 850 SF

If there is sufficient equity in the property you would also be entitled to be reimbursed for any property taxes you pay. 

If the resident doesn't have fixed lease and has month-to-month only, simple 30-day notice about ending the tenancy is sufficient without any explanation.

I agree with @Scott Trench, I have been quoted 5-20% owner occupied for residential MF as long as it meets the self sufficiency criteria in MA.

A reasonably sufficient amount here is also a safety margin.4) What I can pay for the property initially. 1 - (2+3) = 4Has worked without fail for over 20 years for me