The SUB files, in combination with an IDX file which you should have also extracted, contain graphical and timing information for the subtitles. I recommend using BDSup2Sub for editing them.

Once you load them in BDSup2Sub, you can set an output format (could also be SUB/IDX or something else, whatever format your DVD authoring software prefers), then go through the subtitles one-by-one and mark them as "exclude from export" as needed--sounds like in your case that's most of them. Then you can export a version of the subtitles with just the subtitles you want. Also, you can manually adjust the timing of each sub before exporting it.

Honestly sounds like a complete PITA to me, but I suppose I've done a lot worse.

EDIT: As for "forcing", from what I've been able to figure out, that's handled through DVD menu logic, forced subs do not really automatically display, the DVD menu logic just makes sure they're always displayed when appropriate. At least I think so... so if you're doing a menuless DVD, your parents will have to manually select the subtitle track using the "Subtitles" button on their remote. If you're doing a menu, then it's doable, but, well, that's a question for someone else with DVD menu experience...