@beyerch You may be interested in a project that is currently being worked on.
This will allow you to read/write your PCM for a pre-CANBUS GM vehicle. All of the issues you mention have been addressed. They are working with a 411 PCM (512k LSx PCM) but the only difference for other models should be the kernel.
The M2 will have a sketch available that will be compatible with this as I work on building a new more modular firmware for the M2 in general. Once totally perfected, some members of this group will be looking to move on to CANBUS based PCM read/writes.
To Actually program for these will require using TunerPro which is a “begware” program. (Will work for free but to get rid of the nag notices you need to make a donation.)
So shortly you will be able to program your pre-CANBUS GM for the price of your interface. Currently working or almost working with AllPro, Scantool, MDI and AVT 832 devices. In the coming weeks I will be working with them to get the M2 basically working with it as I work to overhaul the over all structure of the M2 firmware. This will be a stopgap Sketch until I have something more concrete accomplished. Note I am not an active part of any part of that project yet other than tinkering with the M2 side of it.
Note I don’t have TunerPro and am not familiar with it but it is the program I was told they plan to use.
The program they have to do the programming is to be open sourced and there is an android app being worked on to accomplish this as well.
The Open Sourced software is actually written in .net so it is a windows only environment for now.