"One thing I read somewhere is that if your pressure switch has two connections, it needs to be grounded externally. Is this not always true?"
Yes and no.
With the wiring you have it will work just fine the way it is (so long as teh switch is still good and you have cut the wire at pin D).
The way the pressure switch works is that when it senses enough fluid pressure, it merely closes the contact between the two terminals. There are some pressure switches with only one wire. In these cases it means that when the switch closes they create a direct short to ground..that is NOT your case).
Your TCC is a one wire. That means that the one wire must be activated by +12 BUT internal there is another wire that you can't see..that goes to ground. Remember that there are many ways to wire this system. What one person says out of context may not apply to all situations.
Hope this helps!!