The question seems slightly disingenuous to me. It purports to be about uncertainty in the licence version that applies, but in a specific case: SE's forced relicensing of all SE sites' content. The ambiguity here is not the version which is declared at distribution time, since that is clearly indicated. Instead, it seems to me that the heart of the question is
a) whether content contributed under CC BY-SA 3.0, with the contributor futher granting certain unlimited rights to SE "in order to provide the Services", can be relicensed by SE for general distribution under CC BY-SA 4.0, and
b) if it cannot, which licence should people reusing content from SE assume applies, and label their reused content with.
While there is uncertainty as to the answer to part (a), it is not an uncertain point. Either the terms on which contributions were made permit the relicensing - in which case it's now under CC BY-SA 4.0 - or SE has no permission, express or implied, to do it - in which case all contributions made prior to the change are under CC BY-SA 3.0. So I don't like any of the attribution options listed above.
Unfortunately, I'm no lawyer, and it would seem to me to have got way past the point where they should have been involved. I agree with the point made in the leading answer to the linked meta.SE post that, had SE any formal justification for this, they might reasonably be expected by now to have presented it, and that as the ones trying to initiate the change the onus is on them to do so.
So proceeding on the assumption that the relicensing was ineffective, I turn my attention to question (b). There is further ambiguity about when the licence change for newly-contributed content was effective: this post says that the ToS change to CC BY-SA 4.0 was made 2019-04-30, but those ToS as I read them don't specify a CC BY-SA version number at all. Moreover, the linked post noted that the individual site footers - which is all many contributors see - kept displaying "CC BY-SA 3.0" until 2019-09-05 by "oversight". That further blurs the status of contributions made between 2019-04-30 and 2019-09-05, though I lean towards concluding that contributions on individual sites were made under CC BY-SA 3.0 until 2019-09-05.
CC BY-SA 3.0 provides in s4b(ii) that derivative works can be relicenced under later versions of CC BY-SA, so posts originally predating the relicensing that have received substantive contributions after 2019-09-05 (and thus under CC BY-SA 4.0) can reasonably be taken to have been so relicensed.
It seems to me that any honest agent reusing content entirely predating 2019-09-05 should label it, on reuse, as subject to CC BY-SA 3.0. Content entirely postdating 2019-09-05 should be labelled CC BY-SA 4.0. Content predating 2019-09-05 but with substantive contributions after that date can reasonably be labelled CC BY-SA 4.0.