I would strongly disagree with you about the change in costume since 1912, there has been much more of a trend to become less formal. That is backed up by my acquaintances in the "rag trade". As you point out, the costume has dropped the firearms and so forth. Also, People are wearing PC, rather than doublets. Some folk try to get by with black argyll jackets.

I would look pretty silly at a Basketmakers Guild Banquet in my montrose doublet, but would be accepted in DJ. I've seen people in Suits at these occasions. Usually, they are guests at these events. The standard of dress has gone WAY DOWN in the past 100 years.

I would suggest full formal Scottish attire if you are going formal, but that picture is a bit over dressed for most people these days. E.g., Fly plaids are considered optional. I have a Montrose doublet and jabot, but I think that would be going to be difficult to get the outfit together quickly. Just the tartan hose is going to take a while since I don't know of any place that sells them off the peg.

Mixing white tie elements with Scottish attire is also off.

I'm glad we agree on when appropriate to wear Highland wear.