If raise the rates to the 2 BR rate and earmark the extra rent towards doing the 3 BR Reno whenever they leave.

Buy an extra couple of boxes for inevitable cracks after a while.

I recently discovered the concept of AirBnB, and think it may be a good way to earn some rental income with my primary residence.

If they are late, they pay extra.   

I've raked up a backyard and filled up 4 full sized 50 gallon trash bags of poop after a tenant only being there 1 year (so yeah, I've seen some sh**,  pun intended).The dog isn't necessarily a deal breaker, but you should for sure get a decent sized non-refundable pet fee up front, plus an extra monthly fee that will cover the renovations to put the place back together when they leave. 

A recommendation would be to start setting some money aside now to build up your repairs/CapEx fund and then whatever extra you get from the rent, put it away for future repairs. 

This may require increasing his income for a short time working extra and cutting back on non-essential lifestyle things or improvements to the property, which isn't his yet.

I think the fact that I can put down the extra cash every month might be blinding me to the fact that I shouldn't.

So you have to weigh that.You paid your uncle 10k, which allowed you to to earn 21k.So, depending on your deal, even 15% on 75k may be acceptable to you based on the circumstances.

I have no landlord experience and know that PMs usually charge 1 month's rent to find you a tenant and of course any repairs or maintenance that may ariseWith the following information, would it be better off to stay at my current apartment and just end my lease when it's time to take an assignment or should I purchase the duplex and possibly make extra income through cash flow and equity but also run the risk of repairs, maintenance, etc?