If they were real people then it could be argued that Rob and Ursula are now very confident that they're teflon-coated.
I don't think Rob and Ursula think in those terms. They do what they do because it comes as naturally to them as breathing. They have isolated Helen because they really do think they know best on all points. They are unable to tolerate dissent, so anyone who does not bow down in front of them is best swept aside - e.g. Kirsty. Rob is also very snobbish, sexist and homophobic; we haven't heard whether Ursula is, so it's possible he gets that side of things from his father, but Ursula is still married to Bruce, so must have decided to tolerate those views at the very least.
Thus, in Robworld, he must also keep Helen away from anyone who doesn't match his idea of a suitable friend - so that's Ian ruled out, for being (a) gay and (b) originally from a lower rung on the social ladder than Rob. Adam is to some extent protected by being an Archer (by birth) and being rich and powerful as well as a blood relative, but covertly Rob despises him too and has done his best to hurt him. He had similar feelings towards Charlie, which he was able to give voice to more openly because Charlie wasn't an Archer or a relative, and he believed he had a grievance against Charlie.
It's clearly inconceivable to him that anybody as high up the pecking order as he believes he is (and therefore by association his wife is) could want to be friends with anyone from the lower orders. So that rules out Emma and Fallon, who of course are also negligible because they're young and female (and therefore weak and silly). His barely concealed contempt for the Grundys and Carters and his toadying towards the wealthy and powerful (e.g. Oliver) is one of the things that really makes my hackles rise.
It occurs to me that a Rob/Brenda confrontation would have been good value. He wouldn't have rated Brenda at all, but (when the SWs remembered) she had more brains in her little finger than he has in his whole body.