I don't think any of the episodes or movies have ever gotten higher than a PG rating
I remember noticing quite a few voyagers were M rated upon hiring them out here in Australia
The reason why TOS didn't get as many reruns in Australia is because 40 episodes were rated "G" in the 60s. 39 were rated "NRC" ("not recommended for children"). "The Man Trap" never aired in NSW until 1982. In the UK, four episodes were never screened, and some British fans hadn't seen those until home video.
In the 80s, the network with the renewed rights to TOS repeats
elected not resubmit them to the Australian Chief Censor. Had they done so, most would have been cleared for "G" viewing. So everything got one rescreening in prime time, then the 40 G's moved to weekend afternoons and high rotation.
TOS in the 60s in USA was very popular with university students, who watched in crowded dorm rooms (as legend has it). By the time TMP came along, these fans had found each other, gone forth and multiplied, and raised their kids on TOS reruns.
How did they explain Ensign Cutler not returning in-universe? The actress died IRL and, of course, couldn't come back.
They didn't explain it. We know she was still aboard during the Xindi mission, since there was a reference in "Rajiin" to Cutler breaking her arm off-camera. So presumably she was still there, just not seen. The books have established that she was aboard as late as "Demons"/"Terra Prime" (although
Rosetta mistakenly calls her "Nurse Cutler")...
IIRC, there was an intention to do a tribute episode or scene to the actress who played Elizabeth Cutler. They had paired her with Phlox, and liked the chemistry - I guess why "Rosetta" refers to her as a nurse? But the timing didn't work out, plus Trip's sister was also an Elizabeth, and the centre of a major ongoing storypoint, so they just let the Cutler character slide.