I’m just thinking out loud ….....
From the census we can see that Alexander was quite a bit older than his wife – about 10 years. So that puts his birth more around 1805, therefore I suspect that the one you have for 1815, is not right.
The other thing to note about the census is that his wife Sarah/Marion was born in Glasgow but is always recorded on the census as born Morven. We know that is wrong, so do we believe that Alexander’s birth place as Morven is correct ?
Why the Catholic baptism of the children? Rather strange, as Sarah MacPherson was not baptised RC. Sarah and Alexander’s marriage was not RC. Was Alexander Catholic, and if so, how does that affect a search for his birth and death?
I’ve mused on those things too.
1. Alexander’s age/dob - we can infer from the 1841/51/61 censuses he was born around 1811, so, given general looseness with ages, the 1815 baptism seemed a possibility. It’s the 1871/81 censuses that throw this out - and Sarah’s supposedly being younger.
2. I agree about places of birth. It’s possible that Alexander, like Sarah, was born elsewhere.
3. Sarah’s baptism in Glasgow was RC and their marriage CoS. The likely explanation of their children’s RC baptisms was pressure from the owners of the Drimmin Estate, Lord and Lady Gordon, who were fervent Roman Catholics. There’s reference to this on the history section of the Drimmin Estate website, as well as more detailed background, including on Aulisten, where they lived, being cleared, in Gaskell’s Morvern Transformed.