Pbft,
You are still missing one tiny thing I wrote about earlier.  Tool length offsets are relative numbers to each other.  As an example, if I measure the top of a a part using my middle finger and my pinky is 1/2 inch away then measuring another part with my middle finger my pinky will still be 1/2" away.  If you call G54 and set 4 tools on top of it, and use any one of them to set Zero, then call G55 and and use any one to set Z zero, all your tools will work properly in both locations.  the key here is you needed to pick up Z zero with G54 active on the first part, then pick up Z zero with G55 active on the other part.

Hope this helps.

my machine zero to plate offset isn't as repeatable as I'd like

CNC is only as repeatable and accurate as YOUR machine and how one sets up for a job.

Why not address / fix the machine so you can do what needs to be done?

RICH