Totally agree. Pauline's reaction rang the most true. As she told the school clerk, wives get replaced by younger women all the time. In Pauline's case she was replaced by a contemporary - one she knew intimately. Plus she had just extended an Olive Branch to Liza and done so in a public way. No one likes to be lied to and played for the fool. Adding insult to injury, Pauline was looking forward to having Liza play a bigger role in her daughter's lives. In light of all that, her playing the Nuclear Card like that makes total sense.
Equally important, the writers are now off the hook regarding Liza's age. Sutton Foster is a lovely woman, but the 26-year old charade was starting to look impossible to maintain.
Some of the writers' best work IMO. From a psychological standpoint, that kind of betrayal and public humiliation can rock even the strongest person to their core. Diana was doubting herself, questioning her judgement, wondering how she could be so easily fooled. That was enough to make her (temporarily, at least) switch off her fundamental "Diana-ness."