(2) I think the Support response, presumably worded a bit differently, is quite fair. Such a question could take real developer time to tackle, and, at a minimum,there would be a modest expenditure of resources just in communications between developer(s) and Support.
(3) I did send a link to the person I suspect is most knowledgeable about this. Which is zero guarantee of any reply.
I did know about those. Unfortunately they’re all about implementing the trie data structure in Mathematica, not building one of these .trie search files the FE uses. My assumption has always been that a .trie file is not a .mx dump of the data structure as built in the first of those links, but I could be wrong.