Jess,
screenshots attached.
That makes things much more clear. Thanks!
You're experiencing difficulties because you're not telling PM to Always create and/or update XMP sidecar files. So Lightroom isn't seeing ratings or color labels. Change the popup menu currently titled "If XMP sidecar files exist, add and/or update with IPTC4XMP" to read "Always create and/or update XMP sidecar file with IPTC4XMP" and that should fix things. You'll still need to do some sort of operation in Photo Mechanic to cause the XMP sidecar files to be created. This could be as simple as Tagging and un-Tagging the files, or applying an IPTC Stationery Pad to clear some unused IPTC field.
You should also change the read order for your TIFF-based RAW files to read XMP sidecar files first, embedded IPTC second, and embedded XMP third.
Once you have made these changes, 4.6.1 should work quite well for you and Lightroom should see your ratings and color labels. For best compatibility you should use the same color class/label names in both applications.
-Kirk
