No Robert here. There is no way you can predict or plan for all the variables you're likely to encounter with a large group of dowsers. They all have their own variables, and they are all in the mind. Just like your inability to dowse E/W; that's another autosuggestive result that you've imposed on yourself. It's probably firmly embedded by now so there is no getting around it. But, again, it's all in your head. It's not real.
Do you think that science has not investigated dowsing?
You cannot begin to imagine what dowsers have considered. Location, time of day, phase of the moon, local tides, planet alignments, temperature, humidity, sunspot activity, what they had for breakfast, drinking just the right energy drink, can't wear wool, can't wear nylon, can't wear rubber soles, must wear rubber soles, there's a skeptic nearby... I can keep typing all day. I call these "alibis" because it's what dowsers do to explain repeated failures.
Here's the funny thing. Groundwater dowsers as a whole don't tend to use a lot of alibis because dowsing for groundwater is statistically highly probable. That is, dowse a well location and it's pretty certain you will hit water. OTOH, treasure dowsing has a near-zero statistical probability so treasure dowsers are especially attuned to the long list of alibis. And they really truly believe the alibis for their failures, just like you truly believe you cannot dowse E/W or with galvanized rods.
When I've designed a dowsing test for an individual I let them list all of the self-imposed restrictions and I design the test to avoid those issues. For instance, for you I would design a test to be N/S-aligned. If you are going to test, say, 20 dowsers then send them a questionnaire ahead of time to root out these restrictions. I totally agree and have built some of that into my plan and, now, will incorporate more of those criteria.