HTTT has some frustrating diplomatic issues...

[Carewolf2]

No, that's not true. You get the Ally CB when your ally is at war with someone and you are not, you got it briefly as your alliance status and CBs are only checked at the end of the month. I suppose you could "exploit" this and DoW with the ally CB, but it doesn't allow taking provinces etc.
If your ally has a valid CB you don't get a stab hit for joining.

Well, he didn't have a CB, so honouring cost 2 stab, but dishonouring meant I could still declare war, but now much cheaper and as a leader (ally was a larger nation). Sure taking provinces was more expensive, but without claims it is pretty much as good as it gets.