I'm considering buying a property in Southern Maine that is about 5,000 sq ft too small to legally subdivide per the town's zoning rules.

This is legal and will get the job done.

What are we legally required to prorate/deduct in rent considering it is their only shower and will not have one for 3 days?

Harassing your tenant on being late, when they are not actually late will end up with him having legal action against him.

Quote from @Kathleen Park: I recommend you educate yourself on "ordinary wear-and-tear" vs damages, depreciation, and your state laws.Do yourself a favor: buy "Every Landlord's Legal Guide" by NOLO.

Are the 2 bedrooms in the basement legal?

Is there a legal timeline to update current investors about a significant change in the investment?

And of course, I should note that I'm not an attorney so don't take any legal advice from what I say.

I bought it off market at a great deal with a conventional mortgage as a primary residence with the plans to rent it after the 12 month period has passed so I can legally rent it.

Local Regulations & Market Conditions:In areas where vacation rentals are regulated or oversaturated, factors like legal compliance and market competition can have a bigger impact than ratings.