Yes Shikoku is obviously plotting something, but part of the trap is that Ren can't just attack her even if she does suspect her. Attacking an envoy raising a white flag will only reflect badly on her and the whole corps, and I wouldn't be surprised if the Gods of Thunder then tried to use that as ammo and start a smear campaign to undermine humanity's faith in the corps and cause general chaos.
So, Ren will naturally be wary, but still hear what she has to say. As for Shikoku's goal, I agree that this is likely yet another ploy to build up Ren's ego higher and higher so that when it topples it'll create as much destruction as possible.