The fact that they waited until the day before the rent was officially late makes it sound like they were testing you - looking to see if you came by anyway to collect the rent in person. 

Language like "obtain more information/history on xx" is common.

Just finished my first official flip and I accepted an offer on the house in December 2020.

Is adding a third bedroom going to be worth it or should I just make it a "bonus room" and not worry about making it an official bedroom?

I'd suggest checking your lease to see if there's similar language that addresses this particular issue.

I am happy to announce that I have officially closed on my second multi-family property in the Middletown,Ny area. 

According to inspectors and officials in my town (also in Bucks), wet basements are very common due to the shallow bedrock and ground moisture.

I would use that language to reduce the odds of the thought in their mind of you not being able to do thatBring in a third financing partnerPurchase the house with owner financing and negotiate favorable terms

Yes, being able to break the language barrier is certainly key!

We are both new to this partnership and nothing is official so odd moments is to be expected lol.