I can now answer my own questions. I was there for a week last month. I didn't take my DSLR and lenses, I only used my inconspicuous little Sony Cyber-shot Dsc-RX100, which is just as good most of the time except for the limited focal length. Photography of trams is acceptable and even encouraged to the extent that drivers will occasionally wave and hoot as they pass, and in many places when you are overground the lines are widely used as public footpaths so access is never an issue.
I visited Alacant Terminal on three separate days. I bought returns to Sant Gabriel (10 mins), Universitat d'Alacant (7 mins) and Elx Parc (about 30 mins) for a few euros and spent a couple of hours on the suburban / medium distance platforms at Terminal before or after my journey in full view of security staff without issue. Just remember to tap your ticket out and then back in again at un-manned and un-barriered destinations or the barriers at Terminal won't let you out when you want to leave (though they were just opened manually for me when they'd checked my ticket).
The far end of platform 9 (over 400m long) is a particularly good vantage point, you can get a good view of all the comings and goings from here, both suburban and long-distance. Hat and water recommended.
I bought paper tickets from the machine by the gates twice, the third time I tried to use tap-and-go with a UK visa card which worked ok in principle but not in practice. The first time I tapped the barrier let me in, then I got a text from my bank reporting suspicious activity and asking for confirmation, which I gave, then another text telling me to try again. By this time I was on the train! I tapped out OK (apparently) at my destination, then in again, but at the Terminal end the barriers wouldn't open. Again they were opened manually for me when I pleaded ignorance. Later that evening I checked my bank account and found that two attempts by Renfe to extract the correct single fares had been declined by my bank.