After transferring photos to LR for editing [...] does anyone know what I need to do to make the photos go neatly back to the folder where I was originally working in PM? Thanks!
When you transfer photos to LR for editing, do you select "ADD" instead of "MOVE" in the LR Import window? This will add the photos to Lightroom's catalog without moving them to new folders. That would be my first suggestion. Note that your Lightroom edits won't be visible in PM
until you update the metadata in Lightroom.
Another thing you can do is, when you export photos from Lightroom, there is an option to open them in another application after the export. (Way way down at the bottom of the scrolling Export dialog) If you choose Photo Mechanic, then PM will open a contact sheet for the newly-exported photos no matter where they are. It can be pretty handy in some circumstances.