Much of the regulation applies to renting out individual rooms and/or short term rentals in an accessory dwelling unit(attached or detached ADU on the property. https://www.oregonlive.com/portland/index.ssf/2018/08/80_percent_of_portland_airbnb-.htmlAs a side note,  CM2 land on SE Belmont is a prime zoning for building plexes. 

Are there restrictions/regulations on what an owner can do to a commercial property?

Abandoned property laws may still apply, depending on your state regulations.

What's your method of gathering information on regulations?

Some thoughts:I suspect your realtor was referring to CA ADU regulations but they do not apply to duplexes. 

The large banks will often have their own overlays which are more restrictive than the pure fha regulations.  

I don’t know of any law/regulation that requires them to allow you to simply bring the loan current, and make payments.  

Do either of you know if anything changed for the Madeira Beach vacation rental regulations since 2 years ago?

When I renew a lease, I do get price up: the better the tenant the less increase.If your state doesn't regulate the increase, send them new lease to renew with new market price.

I did have my electrician do an electrical plan overlay for the electrical in the kitchen, so that is one factor.I will also say that there are crazy energy regulations in place in CA that can impact the electrical and HVAC.