just registered because I want to tell you that it's possible to make it working, as shown in these pictures:
The point is:
when you change numPerMulti you never said the underlying window to resize, and it wont, unless specified.
I have done 2 tests:
looking at xsd file it's clear that the main window can accept the resizeOnChildren variable, so:
1st test: i just changed sizable variable to "true" in "inventory" window.
2nd test: I tried to use resizeOnChildren="true" in "inventory" window leaving sizable="false".
I used in both cases numMulti="7". It works.
Of course, i didn't got deep to make changes for the coin control, but obviously, with some work, can be done.
The same thing can be applied for the Bank window but I don't know if it can be applied to other bags.
I think you have enough info to do more research now
I hope this helped you.
Edit: would be nice to have them automatically rearranging with resize, but seems you have to specify numPerMulti. I tried to remove it but, ofc, i got a single row. I think SOE just need to fix UseMultiRow to be as a "word wrap-like" feature.