Radon is a single atom gas and so it penetrates plastic a little easier than other gases like CO2 but I can’t imagine there’s any reason it would degrade rubber. A good rubber gasket should last _much_ longer than 5 years since it’s exposed to basically nothing. You wouldn’t replace rubber plumbing boots that often (I’d wager most never have!). 

Yes, freezing and condensation is what drives the Canadian codes (and probably should in much of the US, too) but the point is that sidewall venting causing back draft in to windows obviously isn’t a very serious risk. If it resulted in substantial cancer rate increases then the codes would instead call for roof venting with insulation, backdraft dampeners, etc as we do with natural gas flues (because CH4 needs to go out the roof whether we like it or not).

I think the EPA guidelines of 4pCi/L are difficult to substantiate in the scientific literature but you were definitely fine at 3.5, too even under the EPA guidelines.