As I posted previously all the internal gear hubs have limits on how low the gear ration can be. You will have the same problem with the other Shimano hubs. This is printed in the Shimano sales and support manual. If your shop does not have them they can just ask. I have some for different years. The most recent one I have is 2011. They list the minimum size chain rings, because of torque problems. This is very old news. Don't use any of them for a mtb. Period.

Your assumption about 1.6 not being too far from 1.9 and 1.9 not working is wrong. All this stuff is documented, and available.

I'm amazed your shop did not know this !! This stuff is decades old. Same for Sturmey Archer hubs. Plenty of people have used low gear ratios on IGH and ruined them.
Sheldon Brown writes about this too. It's easy to find on line.