If you really get this integration to work and you can synk your efforts with DXO there will be more people who won't hesitate to leave Adobe and Lightroom for good. You have all the reasons to solve this since Photolab and PhotoMechanic is an extremely powerful combo. Maybee it's DXO and PL that has these limitations but since there are two of your softwares interacting you might have to talk. I would be very glad if your people at Camera Bits could sort this out together with DXO.

If I have both PMP and PL open and select say five images in PMP and chose to edit them all in PL with the Edit-link in PMP Photolab it only opens one of them BUT if i expose both windows side by side and drag and drop them from PMP to PM it works with all five images.

BUT, there is an issue I have tried to get you to solve: Photolab kan see the metadata added by PMP without even an import and if I export from PL to a DNG or JPEG everything works as expected what I have seen I use PhotoMechanic Plus with Photolab 4 and I see no problems that I can't handle with the integration.

Capture one vs dxo photolab 5