My thoughts are you are pretty much right but I would say as soon as matching panel has happened you should be getting going on introductions. It is a shame matching panel cannot be brought forward a week or so and things move quickly after that.
Presumably the children know this is foster family is not their forever family and so already know they will be moving on one day.
My ds was just over 3 and a half when he first heard about us and a couple of weeks later when he moved. He did not have final contact with birth parents until after matching panel.
We sent a DVD, a book of photos and a snuggly before we met him and in the space of less then two weeks the family finder was able to do a brilliant job of preparing him to meet us.
Be aware that the introductions will be in the foster carers house etc so my personal feeling would be to do this as soon as is reasonable, e.g. not waiting until the foster carer is just about to move and their home is surrounded by boxes of packed up stuff!
I am sure it will all be quite emotional and hard for foster carer as well but I cannot see how it is at all in the best interests of these young children to wait.
Good luck, whatever happens I would imagine it will work out but I agree you are talking sense in this situation. It will be so confusing for the children to move twice in such a short time.
At the end of the day you can only make your feelings known, politely, not being too aggressive (even if you feel like it, I know I would
).