but it looks like there’s still a lot of animosity between the teams

Wouldn't you be kinda pissed if you had to essentially re-design your car AFTER the season began? :)

(from what I read last night in passing, Ferrari et all will have to essentially re-engineer their cars ... it's not as a simple as modifying their existing diffuser)

It seems a lot more personal than just being annoyed at being behind in development.

Yeah, whoever lost the decision has a big redesign ahead of them. At least this way the redesign is optional - no-one's forcing them to use the new style, whereas if Brawn/Toyota/Williams lost they'd have no choice.