Countdown is a show where having no audience wouldn’t make much of a difference to the atmosphere whereas a show like Have I Got News For You and The Last Leg would be affected more by a lack of studio audience.
It does affect it if nobody can answer the Conundrum, they throw it to the audience before revealing the answer. Minor, though, compared to a lot of other shows.
Not really that much of a problem. They can always ask the crew, Rachel, Susie or the DC guest, or even just re-ask the contestants after the fact, or as indicated above, just instantly have Nick roll the conundrum after time elapses with no answer. Been a fair few times in the last year where Rachel or the DC guest have managed to solve the conundrum if the contestants did not, so not likely to be surprising.