Hi, Mac!

Wish you were closer; we could use another Donnchaidh in the ranks...!

To your questions:

The belted plaid is appropriate for any Highland impression from the mid-1500's up to and including the `45 Rebellion (and beyond that for military portrayals). As for putting it on, we're all still a bit unsure as to how exactly they did it "back then." There is the common (current) method of throwing it out full-length upon the ground, pleating it, laying down on it and belting it on. But, new thought on the matter is that many plaids had small loops sewn in and a drawstring through them to make donning the garment easier, and without having to lie down.
As for casting off the plaid before a battle, I think that notion comes from a specific clan fight in the 1500's that took place on a very hot day, where indeed the clansmen fought in their shirt tails - and the battle became noteworthy for that fact! I tend to think they mostly kept them on. Morier's famous painting of the Culloden battle - he used actual Jacobite prisoners as subjects - shows the Highlanders wearing a multi-tartan assortment of plaids, philabegs, jackets, etc....

For the periods under consideration, the sporran was usually hung from the same wide belt that held the kilt up. They were generally either of the drawstring "rob roy" type, or had curved metal cantles (usually brass) for closure, as seen on some modern "dress" sporrans. They were used to carry all manner of items: oats, musket cartridges, coins, etc.

Your doublet would be acceptable for a pre-1700 impression, though wool was the more usual fabric for such a garment....

Hope this helps!