My MIL tried the complaining about her other daughter in law behaviour to me, I made it pretty clear that I wasn't interested in criticising her, and in fact, I agreed with her, so why would I criticise her for things I agreed with her about. Even if you agree it would be better not to engage about him
RE the zoo, another time keep a fun day for your son to people who will appreciate it, ie your little family. He could have borrowed a wheelchair and got out and about a bit, and retreated to the cafe when he felt like it. Zoo trips are so expensive so another reason not to waste the money on someone who doesn't appreciate the venue. If you do similar another time find a free venue maybe a park with a cafe and go off to the playground or to see the ducks and if they want to come with you so be it. Or maybe a children's farm. My inlaws were pretty selfish, didn't like going to somewhere like a zoo or a playground where there wasn't much of interest to them, once the FIL asked where we wanted to go and showed us some info, and I said I'd love to go to x place but he started sulking and we ended up going to his favourite place, as they were both retired they could have gone any day. I discovered later when the children were older, that there was a very famous children's attraction [this was pre internet] and lots of people said how great it was but they never even suggested it.
RE pressie for BIL, you just needed to get home, what sort of a quality visit would it have been if all you'd done there was walk him around, another time maybe post the pressie. Presumably this was in covid times and your baby could have been ill. Would they have wanted to risk catching something from him? What do you think she would say if you apologised again said you'd not been planning to stop but needed to get home as going to BIL was just too far, you are grateful that they gave the gift to BIL but in future, if need be you'll just post it, to avoid upset, You have a lot on your plate with a small child, they should be fitting in with you much of the time, working around your availability as much as possible, obviously, there will be times when you have to compromise.