Some of the "top" agents (i.e. most sales) focus solely or mostly on sellers (aka. listings), so they will simply pass on onto someone else on their team that works with buyers. 

I simply happen to know the most awesome ones

We simply explain where we are at and why.

And lastly, we all want to do everything in our power to help our clients and customers who come to us for guidance and support but sometimes certain situations are just too complicated to sort out on your own especially as a new agent, although I believe that no problem is impossible to resolve, we must understand that in certain situations it is better for clients if you simply referred them to a more experienced agent.

Those that think otherwise must simply not know the markets very well.

The problem is quite simply it's not the property managers money or property, and the business model of management is at odds with the goals of the investor.

In your case, it seems since you already have an ADU you need to propose an expansion to your main home, and concurrently propose a JADU within the expansion OR simply split your current home into two units equally.

They simply charge a flat fee per transaction and no monthly dues.

If each investor is doing a different deal, then your options are to create a new LLC for each deal with your LLC as one of the members (great idea) or simply have your LLC put together a joint venture agreement with the investment partner.

There are lots of resources out there, a great one is simply to go find your local for-rent advertising sites that will publish what other housing is currently being offered and for what price.