No. Tnetennba is not a real word. It was invented for a joke in The IT Crowd, although the scene has made it surprisingly persistent online.
Yes. A Countdown conundrum can have more than one answer.
I was curious what endings show up most often in the conundrum answer list behind this site, so I counted the suffixes in the current Countdown conundrum generator dataset. For each 9-letter conundrum answer, I looked at the last 2, 3, 4, and 5 letters and counted how often each ending appeared.
I have added two new pages for teachers, tutors, clubs, and anyone else who wants to use Countdown rounds away from the main site.
I recently checked the dataset behind the Countdown conundrum generator, because there are two different questions hiding inside "how many conundrums are there?"
I have been experimenting with exhaustive stats for the Countdown numbers game.