Hi @Jiminycrickets I haven't voted yet, but if I do it will be to say that YANBU. I have to apologise to you as I have only read all of your posts on this subject (as there are now 12 pages) - up to about 5pm when I started writing this - so will have missed it if someone else has brought up what I am about to suggest.
The first thing that really jogged me when I read your OP was when you said
"I responded enthusiastically, volunteered husband to take the group."
I must admit that I was rather surprised by you volunteering your husband, especially at this juncture. Can you copy to us here your exact words on the group chat please? As it might have appeared a bit forward to volunteer your DH to take the group, and they might have wondered why he should take the group - maybe the mum who started the chat wanted to do that herself? I know that it appears on the surface that they could have said immediately and quickly something like "no thanks, we already have that covered, but thank you - or rather your husband - for the kind offer". But how you worded your enthusiastic response may have been miscrued as cheeky fuckery, that is why I think that seeing how you worded it, could help us answer whether you are being unreasonable or not to be upset over this, or whether you are partly unreasonable due to not realising that you had somehow managed to cause offence.
My thoughts are that maybe the starter of the group chat, and a few others, did think it was unreasonable of you to offer your husband for the job of leading the group (I presume you did asked him before you offered his services?), maybe because they are of the opinion that they don't need a man to chaperone their DC, and that if they had felt that the 12 year old children needed a chaperone, one of the mums was quite capable of doing so? However, in my mind, if they had thought that, or some other thing that I haven't thought of, they were being pathetic not to just give you something like the reply I suggested in my second paragraph. I presume if that was the case, that they spent some of the day discussing how to tell you "no", they didn't want that - hence the long silence - and then decided the best way for them to tell you was to get your friend to do the deed, but if that is the case I do think they were being too childish, and should have told you straight away. We humans can spend a lot of angst on the silliest of things, and I am in no means immune to that!
Unfortunately, I have no idea why the other child was not invited, perhaps they felt his mother had committed some misdemeanour too? But of course, whatever their reasoning was, they should have still invited your son and his soon to be good friend too!
Anyway my thinking above is why I have not yet given you a full YANBU, in case the way you worded the offer of your husband could have been mistaken as either a slight of their abilities to chaperone the children themselves, or that they just thought (maybe quite reasonably because of your possible wording) you were being a CF to make the offer in the first place. I suppose if your wording wasn't crystal clear, that maybe you should accept some of the blame yourself, in which case I won't vote, as there is no third option! But they were still out of order to leave the two boys out of the invites, unless the boys were objectionable in some way in their own rights, but that does not seem to be the case here.
I think that if I was in your shoes OP, when I had calmed down, I would like to meet up with the friend who had eventually told me the truth, to try and find out more of what had actually happened, and to reassure her that I don't blame her at all. Anyway, I am very sorry that you and your son, and especially the other poor boy who was more upset about the rejection, went through this upset. Events like this are supposed to be fun occassions, but we humans can be so nasty to each other at times. By the way, you said that the boys who came trick or treating to your house didn't have any adults with them, so I expect they were completely oblivious to the occult goings on!!