How can I fix it while maintaining it weather proof?
I just used a 5mm bolt with locking nut, and a ~15mm wide washer on the inside when this happened to my Ortleib. If you use stainless steel bolt, nut and washers it won't rust and the pressure against the pannier materials stops it leaking. I'd use the biggest washer you can find on the inside, but the biggest I could find was only 15mm. If you look at the other rivet you will probably find that the washer is smaller than that, and possibly that the other rivet is also about to fail.
My repair has lasted several years of regular use, and I still have the pannier today as a backup (I bought new ones when I wore holes in the fabric after more than 10 years of use)
Cut two squares out of an old tube about 1" in size, place them on the inside and outside and use a rivet gun to insert a new rivet. If the tears seems large enough to have the new rivet pull right though you could also use bolts and washers in its place if you don't mind how it looks. But you should be able to get a very near if not water proof seal with the use of tubes. I wouldn't submerge it completely but you get the idea. You may also opt for 2 squares of tube on each side for a better seal as well.