is it possible to lose the de jure right to install the game in that way due to licensing issues on GOG’s end

Someone can correct me if I'm wrong, but my understanding is that no, you can't. When you buy the game, you've obtained a perpetual license to install and play that game, similar to what you'd have if you bought the game on a disk. You can lose your ability to download the game, that isn't guaranteed to be unlimited or perpetual, but installing it via the installer you downloaded, and playing it once you do, are forever. (This is in contrast to something like Steam, where you rely on their servers granting you permission to install the game, and that permission can be revoked.)

Right, if you download the offline installers, then they can't stop you from doing whatever you're going to do with it but you don't own them. Legally, you can't sell them, transfer them to someone else, etc.

There are other sections that make the lack of ownership by you clear and that you still have to abide by the publisher's/developer's licensing agreements but Section 10 states the situation outright:

Section 10 of the GOG user agreement says:

GOG content is owned by its developers/publishers and licensed by us.