As with an unreasonable child, aim to only ever explain, with perfect clarity, your own reasonable decisions just the once. Twice is the absolute maximum as she already knows and is thereafter, simply playing you to get her own way, to show you in a poor light (defence mechanism) and to guilt trip you.
You know that you have been as kind and as reasonable as you can be, given your circumstances which have now changed. Stop doubting yourself on that, you owe her nothing more now.
Of course you will continue to be polite, kindly towards them and reasonable. But you won’t ever be coerced against your will. The line has been drawn by you now, as was your right.
And you must not argue or explain again or apologise or, as you have done, ask for her consideration. She can counter your asking her for consideration by acting as if she has a right to be expecting it back from this day on...conveniently forgetting 2 years of having it all her own way as if you were her SAHM husband!
Quite understandably you will have a desire to only be seen as perfectly reasonable and kind by others she may moan to, for your two years dedicated service to this selfish woman. But she will portray herself as your victim in her gossip about it all to other mums. You can do nothing about that for now. So you just stop trying to pre-empt it. It’s futile. Your own good character will stand up against her reputation with those mums who, like you, take no part in gossip.
I suggest you change your mind set if you can, (hard because you are quite rightly outraged by her conduct) to let this go now and stop investing any more emotion in it. It is already too highly charged and has tipped over into unnecessary drama.
If the little friend turns up before six tonight, then carry on happily and normally under the new arrangement. You know better now, who you are dealing with (a bonus) and of course, you are better off by not having to pick her up. So a job well done.
If she doesn’t turn up then don’t panic.....just assume all is well. Again, you are better off by not having to pick up and it’s no longer your business as to the decisions they have made. Your new arrangement didn’t work for them for whatever reasons. Their prerogative. Not your concern. End of.
As for the dog sitting.... She will try to punish you by not allowing you to dog sit but she will deliberately not let you know that, so as to keep you guessing and fretting for a while. Don’t fall for that.
So just cut that nonsense with an immediate quick text. ‘Something important for us has come up to clash with the date in March where I previously offered to dog-sit, so I wanted to make sure you still have plenty of time to make other arrangements for Bonzo for that date. Thanks’
Finally, as for parcels, let her know when one arrives so that she can come and fetch it as you are no longer able to deliver. She will soon get the message.