Entirely reasonable to expect you to attend. If you want to send a message you are pissed off, disengaged and thinking about options for getting out, then dont go.
It isn't surprising you don't advance in salary if you give off this impression. It sounds like you're doing a good job so why don't you go and use it as an opportunity to sing your own praises to some higher ups.
Lots of mid level people are taken for granted by their immediate boss whilst being valuable to an organisation. When I go to these things I have a list of senior people I want to get around and make sure they know who I am, even if I don't actively pitch them all the good things I am doing.
You sound very resentful that you do a full time workload in part time hours. I don't know what bit of financial services you work in but this is very standard in my area of professional services. Is it the case that you should be managing other people to deliver the work?
If you aren't working 5 days a week I think you have to realise that a full time job would come with even more work.
Just because your other colleague is also disengaged and making childcare related excuses doesn't mean you should also half ass your job.
FTE Salary is good so it's not the kind of job you can mooch in.
Not doing something your manager asks is a good way to get a rep.
Certainly there's a lot of redundancy drives in my sector right now. All of the big 4 are firing and mid tier firms are on recruitment freezes. It's something to consider whether your company is making lots of money after salary costs on your area. You might be glad of redundancy money but equally can you get back into high paid role in your 50s? There's a lot of sexism and ageism about.