To reiterate, the rivets should be stainless (and there are many grades of stainless that are not magnetic, so using a magnet is not an effective way to determine stainless vs. aluminum).
The preference would be to use the next size larger rivet or you could attempt to use a rivet sleeve to take up the extra space from the ovalized holes. Personally, I would not recommend drilling four new holes in the crossbar since having a group of 8 holes right next to one another will weaken the crossbar. Whatever you do, the stainless rivets (especially 1/4" diameter) are going to be very difficult to pull unless you have a powerful rivet gun.
Regarding galvanic corrosion, you are correct. Stainless and aluminum will react causing the aluminum to corrode. If you sail in salt water, rinse off all of the aluminum parts after sailing. You can also put a corrosion inhibitor (Tefgel) on the rivets before installing and this will really help to reduce any corrosion.
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