LIBINDX contains an index to all MIs recorded in 1978/9 in the present version of Moray. This does
not include the parish of Cromdale, Inverallan and Advie, which became part of Badenoch and Strathspey District in 1975 and is now part of Highland Council, who will have any burial records.
Also, LIBINDX contains only details of gravestone inscriptions, not of burials, so even if LIBINDX had covered Cromdale Inverallan and Advie it would only have found them if there was a headstone.
You say that they died in Aberchirder and Longside, but that their deaths were not registered in those areas. It was obligatory to register a death in the district where the death occurred. Have you actually got the death certificates? If not, then that is your first step. You can download images of both death certificates at modest cost at
www.scotlandspeople.gov.ukIf they did die, and you think they might have been buried, in Aberchirder and Longside, you can try the MI index at
http://www.anesfhs.org.uk/databank/miindex/miindex.php If that draws a blank, you will have to throw yourself on the tender mercies of Aberdeenshire Council, who will charge you £10 an hour to look at the books yourself, and probably a lot more for them to look. If you can wait a little longer, there is a rumour that they are going to be on Deceased Online 'soon', whatever that means.
Another thing you could try is to see if there were death notices in the Aberdeen Press and Journal which might say where the funeral went to.