This is exactly my plan at present, just having paid off a substantial holding.

What conditions are present that could interrupt, say, a 20-year run, or make it extremely expensive?

I've been analyzing several deals and I'm looking for a partner to help fund these deals and before I present to them, I want to make sure I actually have a good deal.

I "attended" an online presentation targeted at French investors that promised 16% gross returns, and presented pictures of some properties plucked from Zillow.

It sounds like you have goals - and that’s cash flow, if the market doesn’t present opportunities for that goal, then move to another and/or stay persistent. 

I was on a call with some investors the other day about multifamily, and the presenter walked us through his spreadsheet.

I would also like to acquire some of the properties from her creatively if the opportunity presents itself.My questions are:How do I find a way to add value to this investor and her business when we only spoke briefly for about 3 minutes, and I don’t know where or how she needs help in her business?

So when this opportunity presented itself, it was really a dream come true.

The tenant must be presented with the option to mail in / drop off a physical payment.The rest of it seems pretty boiler plate and shouldn't be an issue. 

Because if I’m paying the seller that mortgage but then they stop paying the original loan back, seems like the original bank can come in and sell my property to satisfy it.Anyways there’s a lot of extreme hypotheticals I’m presenting, just trying to look at it from all angles.