As for your ABBA question, I think it's correct to say they didn't actually appear in the studio again after the 'Fernando' performance you mentioned.
As far as I'm aware, Servalan's provided all of the appearances they made. There were 3 performances for 'Waterloo' as far as I've seen on Youtube (one was April 10th 1974 - this is the clip that's been on TOTP 2 a few times, then again on April 30th 1974, and a performance for the Christmas Day 1974 edition); a performance for 'So Long,' which I believe wasn't actually broadcast because of a strike*; a performance for 'SOS'; one for 'Mamma Mia' and the final one for 'Fernando.'
*EDIT - Conflicting accounts exist for this particular episode. Some say it was broadcast but only in England, others just say it was filmed but not broadcast because of a strike.
BIB 1 - Abba's first ever TOTP appearance was introduced by Noel Edmonds on 11 April 1974 (the week after they won Eurovision).
BIB 2 - Abba's second 'Waterloo' performance was introduced by Jimmy Savile on 2 May 1974 (the same week it reached No. 1); it was featured on TOTP2 last year:
The 'SOS' clip from October 1975 is erased from history although I did see it at the time (from memory the girls were dressed in the same clothes they had on in the TOTP studio for 'Fernando' in April 1976).
When the 'So Long' clip from 5 December 1974 surfaced on YouTube last year it was the first time I had seen it since its original broadcast which I also clearly remember (a discussion started on page 107 of the TOTP 1977 thread about this on 07-06-12):