Lower hook: The blog crazyguyonabike, IIRC, uses Ortlieb, had some problem with the lower hooks that I cannot recall, instead uses a horizontal strap encircling both rear panniers to hold them inboard. And I think he also runs straps going diagonally over the lower corners to support some of the weight, so not all of the weight on the top rack jaws. This is all on a Bike Friday 20"/406 wheel bike, so not sure how this translates to bigger wheel bikes.

When I last toured decades ago, I had typical panniers and everything inside got soaked when it rained. If I ever tour again, it'll be the dry-bag style like Ortlieb or others.

As my townie, typical fabric panniers are fine for shopping and other errands.