
oh wow, I thought it was big but not as much as that...that's a very respectable size.
Just for your information, Montjoye was Legras's top range. A limited production with thick and often acid satin finish glass, decorated with enamel and often gold. The rim that is decorated with gold specks is called in french 'putoisé' and the gold decor is called 'Rocaille'. The irises are a regular theme and here enamelled. It is a very nice example from what I see on the images and you have a serious piece of glass here.
To answer your third question, that's difficult to answer...if only these pieces could talk, right? It could have been transported when people moved country, especially during the war this happened a lot....bought on a trip or maybe Legras did have an outlet in the States at some stage … impossible to tell you with so many possibilities.
For insurance purposes I would not know how this works in the States, but I'm sure you could get information from your insurance company. Maybe they'll refer you to their valuer or a registered antiquarian that they work with. You can get an idea of the going rates on Ebay.
For cleaning purposes, just a damp cloth would suffice. If you do want to use washing-up liquid and give it a good clean, then please put a soft cloth in the sink to place it on and one to clean it with, and for goodness sake, please be careful with the tap. 9 out of 10 pieces get damaged on the rim by hitting the tap. Gold doesn't change colour as it ages, so you don't have to scrub it. If the vase does show some age at some stage, all the better imo, it is in the end a production from roughly 1890-1900 and that's part of its character and again, imo, beauty but that is a personal choice.
It is unusual to see such a piece in this condition and I congratulate you for it...enjoy it, your vase is something that deserves to be looked after.