Hey there.
I looked into this, and after finding some weirdness with plugin order, and my shield block plugin, which I started to fix only to realize it was a plugin load order issue (whoops), I was unable to duplicate this exact problem.
If I leave the crit formula in YEP_ExtraParamFormulas as the default, the MHCrit and OHCrit values are calculated normally. If I change it to a flat value (0.1) then both of them show a 10% crit chance regardless of what's on either weapon, which I think is what should happen in that situation.
I'd need to know what you're using as your crit formula in that plugin to confirm that at least that is working.
I was unable to duplicate the menu display issue at all, though. The numbers changed to be what they should have been every time, and switching away from params, and back again did not cause them to revert to something else, even after changing equipment. I'd recommend disabling the exparam menu fix plugin, as what it does is covered in the DW_MenuExtension, and it's possible the two are interfering with each other somehow. That fix was only to be used if you wanted to show exparams like Mh Hit/crit on the parameters screen without using the menu extension plugin, which changes a lot of other stuff too.
~Ramza