No doubt Wayne will be along soon to provide the definitive answers to your questions, but I thought I would mention that I've been measuring the IR of the UM and other small cells for a while now.
I did Wayne's single-cell and low-current mods (Post #364) which work perfectly (as would be expected, coming from him
I made up a harness similar to what you describe, but for only two of the UM cells, so I could use the Kelvin measurement capability of the meter to eliminate lead resistance error. That worked fine, but I found that I got pretty much the same IR readings when measuring a single cell (using the auxiliary power source). In other words, the lead and connector resistance was negligible compared to the IR of these very small cells.
You do need the low current mod with small cells, perhaps to avoid any possible damage but more particularly to stay within the range of the meter. I use low range for anything under 500 mAh.
Wayne's meter has become my most-used RC tool, and has proved to be the best few hobby dollars I've ever spent. Thanks again, Wayne!