I agree it's confusing!
Looking at all the differing birth places you have, and trying to work logically -
Yes, it says Stonehaven on his death certificate. Information on death records is given by whoever registered the death and not, to put it brutally, by the deceased. Yes, it was in Australia but Stonehaven is pretty clear, and, like Aberdeen, can be a catch all name not just for the Stonehaven itself but also for the parishes immediately surrounding. I would think that is a pretty strong pointer to a birth in Kincardineshire and not in Aberdeen.
1841 census - in a boarding house. Birth places on census should never be ignored just to suit a particular theory but because it was a boarding house we have no way of knowing who actually provided the information. Always bear it in mind but I wouldn't give it as much weight as the information his own son provided when he died.
1851 census - you say he is in his own home so almost certainly he, or at least his wife, provided the information this time. He gives Kinneff. so that has to be counted as pretty accurate and is the most likely option. Kinneff, Arbuthnot and Fordoun are all small villages fairly close together, just south of Stonehaven. Where the border between Kincardineshire and Forfarshire/Angus sat at that point I'm not sure - at present it's south of there, just between Montrose and St Cyrus, so would indicate he was definitely born in Kincardineshire, but I'd do a double check on births in Forfarshire just to be sure.
So taking those three together I'd definitely go with the Kincardineshire birthplaces, rather than Aberdeen. Hope that makes sense!
