My extremely wild guess (I haven't looked at a Jansport pack for many years) would be that there was an add-on compartment, perhaps a zip-off day pack, that fastened on with that zipper and those clips. I really think that it was meant to detach, or Jansport wouldn't have put a tag on the underneath surface. Whatever it was probably added another pound of weight to the pack itself. Any weight in it would tend to pull the pack away from your back, not a good idea. I very much doubt it was for hydration; water is heavy and should be as close to your back as possible. I suspect the former owner ditched that extra compartment or whatever it was a long time ago, with good reason IMHO.

Re #3, a lot of packs have those stupid heavy plastic things to run straps through. They add to the weight of the pack and are not something you'd want to use. Again, anything tied back there would tend to pull the weight away from your back, and would also catch on brush as you hike.

If it were mine, I'd whack off the excess zipper and clips and (if possible without ruining the fabric it's attached to) the patch, in the interest of reducing weight. But, as we often say here, Your Mileage May Vary!

You have demonstrated very well that it's definitely worth searching for gear in thrift stores and garage sales! But make sure that the pack is comfortable for you with a full load before you take it out on a trip.