Hi Tessa,
I must say that I don't see it as Patsy suddenly snapping. I think her stress had built up over several years and for several reasons starting most probably with the cancer. I have no personal or family knowledge of this but I've seen many people commenting that surviving cancer changes a person - he or she can become almost bipolar: overjoyed to have survived one day and depressed and resentful about the prognosis the next. I think she bore most of the responsibility for holding the family and home together and, even with help, her personal standards meant that this was a never-ending task. On that Christmas Day, I believe she was exhausted, stressed, a bit annoyed that she had had a couple of snits with JBR and,crucially, couldn't just chill like everyone else does over the holidays because she has a trip to Michigan and a birthday trip to prepare an entire family for. There is also some anecdotal evidence that the Rameys' finances may not have been quite as comfortable as they appeared and that the marriage was less-than perfect. Equally, if you have been avoiding alcohol owing to liver trouble but have a couple of glasses as a Christmas treat, it's conceivable that you could be pretty intoxicated.
Calling her 'lovely' may have been a stretch and I am not sure that I would have got on with her in RL. The fur coats alone would have got my hackles up and she appears to have taken herself very seriously (eg. The Judith Phillips comment about Patsy not speaking to her husband for a year because he pointed out a spellling mistake or something in one of her written pieces). There's also the suggestion that a lot of her conspicuous kindnesses were 'for show' or from some sort of patronising sense of noblesse oblige. In fairness to her, though, whatever her motives for doing things like writing a last minute Santa poem for Linda Hoffman-Pugh's daughter, she had to be a sensitive person to think of doing it.
WRT her various interviews, I think she was medicated to within an inch or her life. The people who allowed her to go on TV in that state presumably thought that, so long as she wasn't taking a truth serum, there was no problem with her being interviewed.