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John William Mewburn was born 02 Jul 1786 at Whittlesea (the family clusters around Peterborough for some time) the son of Thomas (1754-1795) and Jane (). 
He became a wine and spirit merchant and married Margaret Mardshall in 1811.  They had five children.
I happen to be a Mewburn enthusiast (odd notion) and wonder what your interest in the family is?

I'm sure you've discovered this but there is no sign of your McDonald in ScotlandsPeople.  Strathspey is a generic term and is not the name of a parish, but your man does not show up anywhere in Scotland (mark you there are many parishes where the OPR records had not even started in 1709).  He is not even in the Roman Catholic records.

A subtle point, but his origins would not have been Anglican.  He might have been from a family that belonged to the Scottish Episcopal Church, the principal church between the reign of the Roman Catholics and the ultimate rise of the presbyterian Church of Scotland.  The Episcopal Church in Scotland is quite separate from the Church of England, has various separate practices, and has the honour of having been the source that legitimised the founding of the Episcopal Church in the USA.  These are all now part of the worldwide Anglican congregation, but are not 'Anglican'.  Anyway, if he was Episcopalian that is another reason why he might not be in the Old Parish Registers since the registers were set up by the established church.

On aliases. No one has a definitive answer to them, but they appear most commonly at a time when for religious or political reasons (or both) it could be wise to be vague about your name.  In any case surnames were something that still were not of much consequence unless you were a landowner.  Most of them in the highlands were simply adopted as a sign of fealty to get the protection of the clan chief (though that was already on the way out at the start of the 18th century and collapsed after the 45).  Your McNab may not have any bloodline connection to any son of any abbott (though we would hope for better from the McNab of McNab).

Just to clarify the family position on the Lows I've checked the death indexes for QLD, NSW and Vic for all of Thomson's possible brothers.  NSW is the only one that has any with plausible parents and it comes up with the following:

Registration Number    Last Name    Given Name(s)    Father's Given Name(s)    Mother's Given Name(s)    District
12685/1897             LOW                 JAMES                   JOHN                         MARGARET                            NARRANDERA 
14420/1902             LOW                 WILLIAM                JOHN                         MARGARET                            LOCKHART 
9268/1920               LOW                 WILLIAM H           JOHN                          MARGARET                            GRETA 
9230/1900               LOW                 DAVID                   JOHN                          MARGARET                            KATOOMBA 

John and Margaret are the appropriate parent names and Thomson had elder brothers called James (b1830), William (b1832) and David (b1835).  William H I don't recognise.  This could mean that three Low brothers came out to Australia first and Thomson came out much later after having got his main sugar planting days behind him.  Wish I knew the ages at death for these other Lows.

Jenn, that is fantastic.  The date is reasonable for our man and there cannot be many Robert David Lows around so I think that is a high probability.  It is a shame no mother is listed.  Thomson Low did have an elder brother, James (b1830), but he has been proving elusive in Scotland. I've not found him in the 1861 census but may have him in 1871 - though no sign of a wife at that point (but no reason to suppose that Robert David was legitimate).  Linking father and son and confirming RDLs birth may be tricky.

Have a good time in Sydney.

Ian