If you want to keep pcm and any other module on the bus quite after mode 28 is send, try sending 6c fe f0 3f message every 2.5 seconds. All modules have timer for mode 28 duration and it is exactly 5 seconds.

When you enter mode 34 the pcm have limited time available to upload the code and run it, before it resets. I doubt the pcm will have interupts enabled during upload, it is more a reset after you execute the code the pcm resets.

The code needs to be indefinite loop and the pcm should be able to run and communicate only from the code you send in ram. First thing needed is to disable interrupts, and reset watchdog timer every xx cycles.

A good strating point to look at is the gm main ram loop uploaded at the start of flash. Keeping both processors quite might help with the interupts.

Mode AE is device control mode, similar to mode4 on earlier pcms. It works like sending a special command to add 2 degress of timing or enable solenoids or relays. It is pcm specific what is available for control.

A8 is some gm test mode. There is not much info about it.