While it seems to make sense that the kilt should have evolved from the brat, apparently most scholars think that the kilt evolved from the leine. I remember reading something the other day...an account written about 1540 (1580?) where the English Earl who was observing a mixed army of Celts from Scotland and Ireland could tell the difference between them because the Irish belted their leine's and the Scots did not. No mention is made of tartan at all. The belted plaid (tartan) kilt came into being around 1600 as the saffron shirt went out of fashion or more likely, was legistlated out of existance by the English.
I need to make one correction to your otherwise excellent post. The belted plaid did evolve from the brat, and not the leine. The brat is, as you stated, a wrap or a mantle (blanket) worn wrapped over the shoulders. The leine is a shirt.

Keep in mind that in Gaelic, plaid means blanket, and feilidh (as in feilidh-mhor) means wrap. The belted plaid is a blanker (or feilidh-mhor, large wrap) that has been belted around the body.

The reference you mention is from the Life or Red Hugh O'Donnell writted in 1594. The quote distinguished the Scottish Hebridean soldiers from the Irish soldiers in that the Scots wore their "belts outside their mantles" -- that is, their belts were worn over their wraps, i.e. as in a belted plaid.

Everyone wore their belts outside their leine's, as these were shirts, always pictured worn belted at the waist.

So the belted plaid indeed evolved from the mantle or brat. The articles on reconstructinghistory.com affirm this.

That's a great site, by the way, one of the few out there that can be trusted for accurate, well researched information. I highly recommend it.

Aye,
Matt