The additional expense of becoming an agent is prohibitive for me.

Landlords that use the excuse that turn overs are too expensive usually are either paralysed with the fear of managing, very common of new landlords, or in the case of experienced landlords just too lazy to bother managing their tenants.

@Albon ShawSure they can be expensive , but so can running a new sewer or water line .

If you combine them into a commercial property, the lending rules change and financing becomes more complicated and more expensive.3) the economies of scale are the same for 3 duplexes or a 6-unit parcel.4) insurance is likely less expensive on 3 duplexes5) a valuation based on NOI may or may not be greater than 3 residential buildings.

Lately this person has become less reliable and more expensive.

Yes, ignore the ROI.Yes there is a formula that you use:All income (cash flow) at year's end - all expenses at 1st year's end = Cash on Cash Return        cash you used (cost & fees fro HELOC for 1st year) at start

Avoid those expenses like the plague.

Basically you assume several exceedingly bad and increasingly unlikely scenarios and see what your finances would look like.For instance, look at what your numbers would be if 2,3 or 4 units went vacant and you had large capex expenses like a new roof.

Closing Costs in DE are expensive, would I pay those twice?

The expense you incur is your "tuition"; and modify your process in order to never, ever make the same mistake twice.