Hello, Will, and a lovely good morning to ya.

First, no I don't have any really new cockpit updates or anything. I had the latest textures from Mr. Azframer (for which I am MOST grateful), but they were included in the v15.6 downloads. So, no, nothing new. Must just be different systems, maybe? Textures are FAR from my forte.
If I select a C172 and then come back into the FA18 I see it is now ON. I input 20X in the TACAN and T/R "Transmit/Receive is selected
The answer is contained in the question, as my sensei used to tell me, which I always found bloody frustrating. First and foremost, yes, I use a add-on scenerey from FlightSim.com, zip file name
KNRA_FCLP.zip. Most importantly, though, is that you're still plugging TACANs into the UFC as a TACAN, not a frequency. When you switch to the Cessna and back, it's converted to frequency, then to TACAN. If you put it into the UFC as a frequency, I believe your troubles will be fewer. Instead of 20X, punch in (in this case) 10830 (enter). It's a bit of trouble the UFC has always had. It will also indicate OF until you are in-range, and some of them, you need to be off the ground, sometimes as far as 1200', although the closer you get, the lower they show. I am pretty sure, though, your problem is HOW you enter the input on the UFC, not WHAT you enter, as in, you enter 20X, not 10830. Get used to always entering frequency. Every TACAN has one

As to the TACANs on the DDI, quick, simple, easy, and painless, and you're gonna kick yourself when you hear this

If you look at the lower left area of the DDI, when HSI is boxed (either side's display is irrelevant), you will see a softkey, OFF by default. If you look at mine, it shows TCN. Just hit the button on the outer edge, like you would to display the MENU choice in the middle of the display, for example. First click, you see TCN, second, BCN (NDB's within the display range), and so on. Display range is selected at the top center softkey, marked SEL10 in my full cockpit picture, as I have the 10 mile range selected. Each button press increases the range, until 80 miles, when it cycles back to 5 miles, the default. Every softkey has a use. Play with them. Worst thing you can do is change displays
I hope this finally clears your questions up! If not, let me know, and we'll see what we can see

Mr. Azframer, sir, and Mr. Mattieduck, and a lovely good morning to you both also! It really is too, at least here, anyway.
I found that just changing the sim= line for the [fltsim.0] in the D model's to
FA-18C_FSXBA2015 15.6 Training\FA-18_15.6_flt will do the trick also. Note the lack of the "../" part. Also notice the switch from the use of the front slash (/) to a backslash (\). Does it really matter? I have no idea, but I DO know there is no FA-18
D_FSXBA2015 15.6 Training\FA-18_15.6_flt file, so having the D instead of a C will display an error when trying to find the plane.
I hope this helps a little bit...
Pat☺