Welcome to the forum!
In terms of updating the installer, we will for new customers, but I have no idea I'd we'll be able to resend to all existing customers. I've not seen a way to do that yet. The in game updater will easily update you, however!
In terms of the download, ideally you only need to do that once. If you send a reply message asking for a re-download, someone manually responds to that on our end. It's presumed that unless your hard drive crashes or something (which we understand happens), that generally people hang on to files they want to keep.
One tip: the game uses nothing in the registry or any folders beyond what you installed it into. So if you want a "re-installable" backup of whatever particular version of the game, then just update to that version and then zip up the folder it was installed to. You can run the game from that folder elsewhere on your computer, on a new computer, on a flash drive, whatever. That makes it very easy to archive!
Hope that helps.