If not type up something very simple stating what is happening and you both sign it. 

The simplest explanation is it lower the cost of the sale.

Sounds like your simplest solution would be to partner up with the co-borrower.

Get the most you can for it in the simplest and fastest way. 

They are always ready to go the extra mile for me, and my properties have not been the simplest to manage lately. 

Well, a general partnership offers no liability protection whatsoever, so I would not recommend that.An LLC with both you and your partner as managing members would be the simplest way to protect yourself individually.

(I won't get into the specifics of how exactly I'll be starting my real estate investing journey; just be assured that I have found a highly profitable niche in an overseas market that has very few downsides.)My question is: What is the most important thing, the most useful tip, the simplest fix, your two cents, etc. that you have learned either from personally investing in real estate or through your education and preparation to begin investing?

I'm a Boom Operator in the Air Force; Which is, when boiled down to it's simplest, a glorified flying gas station attendant.  

The latter is the simplest; least amount of tenants, most straight forward. 

Simplest and cleanest, without any potential sibling issues, would be to simply lend them the money, pay off the mtg, leave title as is, do the normal trust or probate process when applicable.