With regards to your experience at college A, what the teacher said was justified for the most part. From their perspective, either you decided to switch subject at the last minute, in which case you should have done the holiday homework anyway, or you decided a long time back in which case you should have requested a switch much earlier. Irrespective of that, it is irresponsible not to complete any work you are assigned even if you don't think you will be continuing the subject further - you had two and a half months to complete a few hours work, it's not a huge ask considering you didn't seek to switch subjects much earlier.
As for switching subjects, do NOT do it because the other subject looks easy. Your other reason, that econ is more respected and keeps options open, is a better rationale. The most important reason though, which you haven't mentioned, is which A-levels are useful for your desired degree courses. I suggest doing some study and finding out which subjects are actually going to be useful for your long-term goals rather than focussing on what is 'easy', 'interesting', or 'respected'.
On the topic of college B, having sixth-form students come in even when they don't have many lessons that day is standard policy as far as I know. I am in a similar situation - I have to come in until 1:40 every day irrespective of whether I have many lessons, and I also live 30mins away from my sixth form. It's a bit annoying but you have to live with it like everyone else. I can't speak for the attendance lady's tone, but frankly there's not much she can do about it if that is the college policy.
I agree that the class cancellation thing was stupid, but realistically as your A-levels ramp up they will give you more priority. It's likely they cancelled your class because they can trust college students to do private study responsibly, but the same can't be said for 11-year-olds. However, if this is becomes a repeating issue, I strongly suggest taking it up with your head of college (or other leading staff member) and asking them to do something about it due to potential impact on grades.
Ultimately the choice of college is up to you but make the decision over consequential points like which subjects you are being allowed to take and how this aligns with your goals, rather than trivial stuff like whether you are allowed to not come in on certain days.