I would probably suggest trying pure silicone grease instead of petroleum jelly since it will probably be more inert and wont cause the plastics to swell further. I happen to have some that came with my ER RS80 skimmer, but you should be able to find some online like at the ER web site:
Euro-Reef Silicone Lubricant
Funny you should mention the screws. My first bracket became so difficult to turn, I thought I was going to bust the screw. I assumed it was because my lower screw comes in contact with salt water. Robbert was sending me a replacement pump for a minor volute defect that was keeping my volute cover from fitting on correctly, so he included a new bracket with slightly looser nylon screws. The nylon screws on the new one have stiffened slightly, but have not jammed up like the old one.
Needless to say, Robbert provides good customer service.
You can buy silicone lubricant at Home Depot, Lowes, or any hardware store pretty inexpensively