I am going to guess (haven't see it) that the likely 1857 death of David Snr was in those early years where registrars simply reported marital status as single/married/widowed. This might add to the uncertainty on names.
Doesn't help Molly that you mentioned that when Robert married Margaret, he gave mother's name as Crawford ?
However, you have Robert Inglis with his family in both 1841 and 1851 in Markinch. He is working away from home by 1861 and then you have him from there until his death up to 1900.
As Annie mentions, maybe have a look at David, father's, DC, for yourself. This is good affirmation of what you have so far. Not sure if it would show as married or widowed. Can't see Mary Crocketts' death reg so far, so this info from David's help would help you know.
Also, if you hare happy with the 1841 and 1851 censuses, have you tries searching on siblings' marriages and deaths. This is additional verification on parents' details.
Monica