They're trying - you might not see it now, but the M+S treats comment wasn't someone trying to obliterate the memory of your child - it was someone trying to start a conversation with you, which might have led on to the deeper stuff, because people generally don't tend to go straight in with things like that - in case the person in question doesn't want to talk about it (because not everyone is going to cope in the way you've laid down by your "rules").
The song again, was meant with the best intentions, while it might have seemed simple and trite to you - it was at the level the children who knew your daughter could relate to... ok so the mum involved doesn't have a career as a lyricist - but they were making an effort to remember your child as best they could and acknowledge that they were going to miss her. They reused a tune that the children knew so there wasn't the focus on having to learn a tune - but the focus would have been on the words and how the children were feeling... perhaps not the most appropriate choice of song, but I'm willing to bet that was the logic behind it - and NONE of that is mallicious. Most things at the level of children that age can appear trite to adults - but to the kids involved they would have meant a heck of a lot. I remember a classmate dying when I was at school - the cards and pictures we drew and made may possibly have seemed clumsy, too colourful or something equally offensive to an upset adult eye - but they were us saying our goodbyes at a level that was appropriate to us at the time (we were slightly older).
You can lash out all you want - but they don't deserve a slap, they don't deserve a slap for the innocuous "good weekend" comment, they probably haven't got a clue how else to phrase the Friday goodbye thing with you at the moment and are trying to play things as neutral as they can for fear of upsetting you further.
No doubt now I'll be bad for saying that - but you're not seeing things coherrently (inevitable) and you're seeing slights and insults where there aren't any there. Other people will grieve differently to you as well - so you can't speak for some standard set of rules how to act, I miscarried recently and while that normality might be upsetting you and you're seeing it as an insult - it was exactly what I needed when I went back to work, to escape from the pain going on in my mind for a few hours... people are different and that's what makes people unsure of how to react - if there was some standardized approach that didn't offend anyone - they'd all be taking it.
You are in fact being unreasonable - but you're in no position to see that at the moment. These mothers are not being shocking - they're being human, and in their own completely inept way - they're actually trying to do the right thing by you... if they're getting it wrong - you need to tell them as calmly as you can, what help you DO need from them because, however much you're hurting, you can't expect them to be a mind reader.