I am guessing you will encounter the same.  

Most, in fact nearly all, of the terrible situations I've encountered in this business were FSBO/DIY sales gone sideways.

I personally did not use an assumption company to assist with the transaction, but that is an option if you encounter a lender who is not too thrilled about helping you through the process.I was able to assume a loan with a 3% rate during a time when rates where in the high 7's! 

While you can try to negotiate anything, I haven’t encountered any broker that’s been willing to reduce the fees.

Based on the size of the building, it sounds like an older residential building that has converted back and forth several times over the years, making it more likely to encounter the issues mentioned above.

sounds ridiculous but a situation ive encountered multiple times. ask yourself how much extra work you want in your life, offsite landlords get put out of business regularly, I've seen it over and over again. consider your options and learn everything but diving in.

They are stating that because the address I want to list on Airbnb is not my primary residence, I only qualify for a non-primary residence STR permit, which restricts rentals to a maximum of 90 days per calendar year.Has anyone else encountered this issue before?

Because it takes repetition for people to remember things and you have to be top-of-mind when they encounter a potential client for you!

The problem is, the inexperienced operator hasn't perfected the ability to buy off market properties at the purchase price that makes sense, so they often over-pay and then encounter less lending options as a result.

I have not encountered what many here have.