From what I understand this compilation special included a clip from the Ethel Merman show. The interesting thing about that is that's the same date that ATV began airing the last three shows of Season One. So that would suggest that ATV aired *two* episodes of TMS that weekend.
Specifically, the Ethel Merman show aired on ATV on April 9th '77 (the last two were Connie Stevens on the 16th and Vincent Price on the 23rd), so as you say, they must have aired two programmes that night - assuming that the information on the back of this photo is accurate; plans may have changed after it was issued.
Unfortunately I don't have the listings magazine for that particular week so I can't confirm what actually happened.
There is, incidentally, no reason whatsoever why Granada - or any other current broadcaster, for that matter - would possess a set of 30-year-old PasBs for a defunct TV station with whom they have and had no connection. They certainly don't own the master tapes for TMS - AFAIK Henson bought those back in the early 80s when ATV reinvented itself as Central - and I'd expect this show, if it still exists at all, to have gone the same way, so asking Disney or Henson themselves might be more viable.
Your best bet - other than buying up old TV Times issues from eBay - would be to try and get in touch with a public library which still has newspaper archives for the relevant period and area. This isn't quite as good as a PasB - it only shows what was *intended* to be shown rather than what actually went out - but it's almost certainly accurate enough; only a major news event or a last-minute strike would have knocked the schedule up the spout
(As an aside, I have an almost complete set of tx dates for ATV - Dudley Moore being the sole exception as the big ITV strike of '79 ended that week and so no listings magazines were produced - and a reasonable smattering of dates for other regions. Most regions did their own thing in the first year, and not everywhere got to see all 24 shows, but apart from Granada they all more or less fell in line with the ATV network feed from Season 2 onwards.)