is this tartan suitable for formal occasions, why/why not?

[figheadair]

I believe the first has a white line in the center of the red ground, whereas the latter has a red ground with no white pivot line in the center.

This is correct although why/how is unclear. The Smiths' 1850 work showed the plain red pivot version but which they called Chief MacIntosh. In his 1831 Scottish Gael Logan says that "The Chief also wears a particular taran of a very showy pattern" but doesn't give a count for it. Apparently it is the version that was registered with the Lyon Court on 9th April 1947.

Perhaps because of the complexity of the sett there are numerous trade variations of what I believe to be a Wilsons' sett (as opposed to the claimed origin of the Crubbin Plaid).