Tiger, if I didn't know you, I'd say just unplug everything- permanently.
Aluminum and stainless make a very poor combination for screw threads; there are severe galling/' thread seizing problems. Even stainless-to-stainless can be a problem; best bet is to assume the pool parts are 300-series stainless, and use screws that are 400-series stainless.
Other fastener materials that ought to work well with stainless, and much better with aluminum, are naval bronze, silicon bronze, and monel. Monel is a brass alloy that has the appearance and mechanical properties of stainless steel- it even looks silvery!
When I need specialty fasteners, McMaster-Carr is on my short list. Great selection, great service- and not cheap!
Coating the threads with noalox, penetrox, or some similar stuff will reduce seizing, corrosion- and help maintain a good ground. Ditto for the contact face of the lug.
Assuming for the moment that the rim is of rather thin material, you might want to attach your lug to a stouter plate, then attach the plate to the rim with many small screws. In that scenario, you could even have the lug welded/brased to the plate!
