OFM if it wasn't for ds2 and financial necessity, do you think you are enjoying job? I just say that because it seems like you are very good at the new job, and you should give yourself a big positive pat for that, and not feel that you have failed everyone/yourself.
Could it be a week of instant meals, and throwing things in black bin liners until after th party? Plan something lovely to do with ds2 after school on Monday (I mean hearthside type of lovely)
Btw, the "weekend family" can be a grisly prospect at the best of times 
You've had a really horrible week dealing with two nasty and stupid people and I'm not suprised it has taken it out of you.
P.S. I've failed to take my children swimmign too 
on a good note, dd has finally made friends with the neighbour's daughter, and they are doing dressing up, with straw (raffia) everywhere - I'm waiting for disaster to strike.
Dss started watching a film called Liar Liar, but dh said it was completely unsuitable, being all about breasts 
They have reverted to much more appropriate Space Chimps.
I have done v. little, except washing and genteel supervision of play. Feeling bored/tense really and trying not to think about tomorrow's op for ds1