There are several Achnahannets (spellings vary) but IIRC only two of them are near or in Strathspey, both in the united parish of Cromdale Inverallan and Advie.
One is north of Dulnain Bridge
http://www.geograph.org.uk/gridref/NH9727The other is further east, north of Grantown-on-Spey
http://www.geograph.org.uk/gridref/NJ0633If James M(a)cDonald was aged 45 when he died in 1882, he would have been born about 1836/1837.
In 1851 there's a family in one of the Auchnahannets consisting of Donald McDonald, 80; daughter Elsie, 22; sons Donald, 16: James, 15; and William, 9. From the names of the other places in the same enumeration district I reckon it's the one near Dulnain Bridge.
This looks like the family of Donald McDonald and Ann Stuart. Their son James was baptised on 8 September 1836, so he is a very good fit for the one who died in 1882.
They also seem to have had two sons named Allan. One baptised on 22 April 1821, and another on 2 June 1842. Presumably the first one died, but if the second one is your disappearing MHP, where was he in 1851? He's not in either of the two census transcriptions I've looked at, or in the census index at Scotland's People. So did he die young, or did he just get missed off the census by mistake, or was he somewhere else and if so where?