6) if I decided to move forward with this deal, or a similar one, should I retain a real estate agent as a buyers agent?

One other factor is that there is a retaining wall that will need to be replaced in the next 5-7 years.

If he does owe money, and you want to look into the 'mortgage wrap' or 'subject to' stuff, then I'd suggest retaining the services of a lawyer and probably someone in Nevada (or wherever the property is) who has actually done that before.

My question is that based on reading the assessor information it appears that LLCs may be able to retain the 4% rate.

In MD and VA, Im not sure what rights the former owner retains, as I havnt had to deal with it on those sides of the border.

You can offer a $5k retainer or something to hire an agent for X months. 

The successful bidder's deposit will be retained at the sale and applied to the sale price.

I would try to keep 2 of them, and retain for long term hold, but I guess that arrangement with your fam member would work out if they're collecting similar rents, and in comparable area.

Do others use an agreement that states the referring broker retains exclusivity of that client when it comes to the future listing of the property? 

I suggest you retain 51% control (as I do) so that someone (you) can make decisions when necessary.