[Autumn]

If this was an issue with simply Spellthief's access to Ring of Pearls then I would have brought that up as its own separate topic, and the additive bonus of Ice Point Guard and Ring of Pearls is completely fine if they do not stack together, I do not believe even wraithguard or shared pain should stack with all of these bonuses but at least those are restricted to main class only.

No I don't believe making them multiplicative is going to be the answer here either, in that sense you're punishing the builds that relied on 1 of those DRs instead of both, which in my eyes so far, only one evasion DR increase has been very good for dodgy characters to just have.

Making the highest one apply is the most elegant solution I feel, its a very easy restriction to understand, especially if its written in the description of those skills.