... remove speedo to repair odometer, but not a budge. I've also removed the mounting plate/dash but it would only come out about an inch, not enough because of the short cable. There is very little access from the back, and by hand I've not been able to put much pressure on the back of the housing. So is there a secret way to remove this speedo without destruction of any parts?

Make sure odometer reset cable is loose.

you can take out the revmeter next to it to give better access to the speedo connections.
Disconnect the speedodrive cable and the tripcable and it comes out further so you can take off the electrical connections (instrument lights)

Mine was no fun because the odometer reset was pulled tight as part of the dealer installed air conditioning.

I tilted mine forward with a towel laying on the steering column, disconnected the wires making a diagram of where they go and then lastly disconnected the cable, it all comes out. I was not so lucky in the repair it failed, I put it back in the car and next year will send it out to the experts. That one was a little above my mechanics level, not a mechanic but do many small/tiny repairs. I would have sent it out now but we are just starting our driving season here in boiling air south AZ