Fortunately, the condo has two showers. a plumber will out tomorrow, but I don’t understand how they find the source of the leak, if I can’t authorize for them to cut the dry wall on a condo that’s not mine. shouldn the owner downstairs call a plumber? 

Fair warning -  it is death by 1000 cuts while you furnish a place to make it market ready.

Should I consider refinancing to help decrease my monthly payment (my current rate is 6.41%), should I cut my losses and sell the property (which I’m hesitant to do bc I don’t expect to househack again), or is there any other route I can take?

Step One: Gave them a taste of entrepreneurship (rite of passage stuff) they cut grass, shoveled snow (as a group) Team workThen my wife an I let them try their own ideas out on the world.

Many people think that they can or should be able to cut out a realtor or broker based on their opinion of the level of effort they've made OR the fact that they went around their backs and tried to negotiate directly. 

While it appears to be a clear cut case of negligence, getting financial relief can be more challenging.

Access was just cut off.

Seems like I have my work cut out for me.  

If you put it out there asking for a 3-5 year lease with a small business or startup in your mind, but H&R Block or Supercuts comes along asking for a 10 year NNN (& it's corporate) and agrees to the rent amount you requested, there's no reason not to entertain that, you got better than what you were expecting (there's a reason NNN properties, or ones with a few NNN tenants sprinkled in that are national chains, often sell for lower cap rates), it's not like taxes or hair cuts are going obsolete any time soon. 

Unless you're on the beach, i'd avoid the flip flops and board shorts, but Also I think most people get defenisve or put off by suits when they are knocking on someones door, so Business Casual, collard shirt if that is something you'd wear or a Clean- Non Graphic T (unless you had your own made), clean cut dark denim jeans -- and some clean shoes.