unless you have some aversion to the bow tie, I'd say it's quite proper and will look dashing. I think that a three button low cut vest looks a bit out of place with an Argyll jacket. I'd stick with the higher vest. A low cut vest, probably in white, is essential for white tie...but then you'd also need a doublet or PC. Your current rig is a bit dressy for semi-formal (though if you lost the tartan hose would be perfect, IMO)...and is perfectly black tie with a turn down collar shirt. Don't listen to those that say "too much tartan". I say that rig is oustanding!

Sporran? Either hair, fur, or leather...so long as it has a chrome / silver cantle...or is the proper type of fur that could serve as either informal or formal.