Lets summarize:
Your symptoms:
1) your car won't shift into drive or reverse
2) you have a black rectangle in your IC, but car can start
3) the error codes you read are P0600, P0715 and P0720

Things done:
1) replaced RCM - issue persists
2) checked all of the CAN BUS wires and terminations - they are good - so probably not an issue with the CAN BUS
3) will check if you're getting 12V to the TCM and SLA

Now I'm just spit balling, not sure how this relates to the CAN BUS codes
a) have you checked the mechanical linkage between the SLA and the transmission - is connected (I guess if the car can roll after you take it out of park, but can't roll if it is in park says the linkage is connected)
b) have you checked transmission fluid level - have you seen any transmission fluid leaking under the car or around the transmission. Note, the transmission cooler is inside the radiator, so you could have a leak into/out of the radiator - is your coolant looking pink with froth?
c) When I looked up the P0715 and P0720 codes just now, they seem to be related to the transmission speed sensor, so maybe the transmission harness or connector coming out of the transmission is damaged - have you inspected it? (There are a lot of posts on how to replace it, although I've never done it myself.)
Not sure why b and c above would lead to a black rectangle IC...

Also, I'm not sure how to check if the SLA is good or not except by using the DRB III scan/emulator tool. When I had my issue, I could interrogate the SLA and TCM to see what gear they thought they were in - that was all good. Again, just spit balling, but if the SLA and the TCM think the transmission is in different gears, the PCM probably won't engage the transmission.