Here's where I'm at thus far.
Yesterday I built two new targets using 918 patches defined in i1P. I made the patches larger (9mm). I saved the patch set so I can open them in ColorPort to build a target there too. What's great about CP is once you import the patch set, you can rearrange the colors manually. Why we can't do this in i1P is beyond me. Anyway, I moved all colors in the CP target such they were vastly visually different from the next. I feared that the original target wasn't being measured correctly due either to too small a patch or colors too close together. I wanted to make sure that as I scan the target, there are no similar colors to confuse the software. Not possible in i1P. The other nice thing about the CP targts is there are white or black lines between each patch. Only downside is it took three pages vs. two for i1P.
Client sent targets overnight, they arrived this morning. Measured in CP using i1Pro legacy mode (yes, it's hard to believe X-rite still hasn't updated CP to recognize a device they released copule years ago). So I'm not getting dual measurement mode, it's M0. No complaints from the software about the data. Saved a CGATs file to build the profile in i1P. It looks fine in 3D unlike the original bad profile that used with more but smaller patches. I then measured dual scanning in i1P which of course recognizes the device as a more modern i1Pro. No errors there, profile also looks smooth. Comparing the two profiles, they look slightly different in gamut but overall shape looks very similar. It's not an apples to apples comparison either, I've got dual scan data vs. M0. Now I need to hear back from client about how the two print. And difference between them.
I'm hoping the original target was the issue on this end. Will report back when I hear from client.