I'm also having a big issue with regards to external track pad support in LR Mobile for iPad. Not so much as it doesn't work at all, because it does, but it does not work in any of the side personas such as within the Masking and the Remove personas. My Apple track pad works fine in the main persona, but as soon as you pop into another all pinch zooming is gone.
My work environment is as follows:
I have my M2 6th gen iPad Pro 12.9 plugged into an Asus ProArt display PA279CRV in the required keyboard configuration. I have my iPad docked in a Magic Keyboard folio with a Anker 565 11-in-1 USB C Hub. From there I have my ProArt disply hooked up VIA Displayport. I use all three external Apple Keyboard, Magic Mouse and track pad.
I just wish that one last thing I need the most would come in the next update, that there would be track pad support accoss all functions in LR Mobile for iPad regardless of what persona you are in.