Gilets décontractés

A casual waistcoat is cut to be worn on its own over a shirt, without a jacket, usually with a fuller cut, patch pockets and a fabric back rather than a satin one. A formal Highland waistcoat is cut close to sit under a jacket and is not really designed to be seen alone.

Yes, and that is what these are for. The rule that a waistcoat needs a jacket applies to formal three piece tailoring, not to tweed and tartan waistcoats, which have been worn alone as workwear and country dress for well over a century. Leave the bottom button undone either way.

A plain shirt with the sleeves rolled, and either tartan trousers or plain chinos. If the waistcoat is tartan, keep everything else solid. If it is tweed, a tartan tie can work. Boots rather than formal shoes. With a kilt, a waistcoat replaces the belt rather than sitting over it.

Close enough to sit flat against the shirt without pulling at the buttons, and long enough to cover the waistband completely, since a gap between waistcoat and trousers is the detail that makes a waistcoat look borrowed. Send chest, waist and back length. Measure the chest over a shirt, not over a jumper.

Yes, on any waistcoat with a straight hem, which is nearly all of them. The convention is old and universal enough that fastening it looks like a mistake. It also lets the waistcoat sit properly when you sit down. Highland three button waistcoats worn with a Prince Charlie are the exception and fasten fully.

For a guest at a daytime or outdoor ceremony, comfortably. A waistcoat over a good shirt reads as dressed rather than underdressed, and it is far more wearable in heat than a jacket. As the groom, have a jacket available even if you take it off at the reception.