The C1231 fault is pretty common on these and is a result of a software glitch basically. Hopefully the calibration they did was a software update to make the system less sensitive to the condition that sets the fault. If all they did was recalibrate the sensors, then they need to perform the software update to prevent it from occurring again in the future. You can reference bulletin TSB-00-86-13.
The P2716 would be completely unrelated from the C1231... it is indicating a circuit fault on one of the shift solenoids (and you should expect some shift-abnormalities from this).
I would want to see with my own eyes that there is a different fault than last time... especially if they didn't do the software update for the C1231 and only recalibrated... they might be blowing smoke because they don't know how to fix the C1231 properly.
Assuming the new code is P2716..........
You are fair to assume it may be a connection issue etc... it is pretty much going to be either the solenoid, the subharness inside the transmission, or a connection issue between them and the main harness. Technically it is possible to have a computer issue, it is painfully unlikely though.
If the problem is coming and going, it is going to be much more difficult to identify. If it is constant however you can identify if it really does need a solenoid or harness using a test meter. You can use the chart below to check resistance.... the top spec being the most important as that is reading resistance across the solenoid.
If the problem is current and the resistance across the solenoid is normal, then you will have a wiring issue or at very least likely case a PCM problem. I would bet the house against that though, they rarely fail... solenoids do a lot.
If the resistance is out of spec, then you would need to open up the transmission to test directly on the solenoid. Of course if it is out of spec the solenoid needs replacement; if it is in spec but when testing at the connector on the transmission it is out of spec, then the sub harness inside the transmission needs replacing.
I've got a pretty darn good feeling they didn't go to any of those lengths to test things and just are recommending a solenoid or solenoid and sub harness in one shot to cover their bases. While it is more probable to be one of those things than a loose connection etc, it certainly doesn't hurt to check the connections and of course before proceeding with any repairs test those resistances to see what's what. Again just remember that if the problem is intermittent, things will test good if the problem is not currently occurring.
I would still be concerned over the code being the same as before or not... again if they didn't do the C1231 update and only recalibrated, the light would be highly likely to come back on for the same thing. I would certainly prefer to have a second place scan the faults to make sure they are being completely honest with you about the code that is set currently.