Tested the KX-T7020's connected directly to the KX-TA824. There is very little feedback when programming unlike the 7030's, but they do work.
Need to check the wiring at a few mod plugs to make sure there are no shorts or crossed pairs.
Once programmed the 7020's DSS buttons on other extensions behave like the 7030's (DSS lights up for intercom, transfer, off-hook but not on incoming CO ringing).
SLT DSS still lights on CO ringing, but I can delay ringing on the SLT which minimizes the problem. I'm going to try to wire the SLT in parallel with a system phone and see what happens.
The KSU and system phones are operating properly and the programming is sufficient for the moment. So I'm almost done!
Sort of an odd-ball problem. I'm just relieved that the hardware does not have to be replaced. Thank you Carl and Alvaphonepro for the suggestions.
