Meaning of 'to make a screen to' (1791 US)
Solution 1:
Yes, you are right that Definition 1.1 (= A thing providing concealment or protection) applies here.
However, I see the "contrast" (and not the "collusion") being used as a screen to conceal the parties' collusion and protect them from public criticism/outcry that would arise from the discovery of their collusion. The thinking being that the public wouldn't suspect that parties holding such contrasting views could ever agree to collude on the matter.