John,
Your comments about Thunderbird behaviour being likely to persist for
some time is enough to make me reach for a glass of red wine.
There's been a patch to restore the old behavior of Reply, but you have to configure it in your copy of Tbird:
"I implemented a preference mail.override_list_reply_to in bug 1392371.
When set to false (default: true) this will restore the TB 45
behaviour. This will ship in TB 52.4 ..."
That works, but restores Drew's original complaint: there's no button to reply to sender rather than to group - one has to copy/paste the sender address. Instead both Reply List and Reply reply to list, which for his issue is not an improvement and is arguably worse since one would rationally infer that they would differ.
The rest of that Bugzilla item states that the new default behavior (ignore Reply-To when List-Post is present) won't be changed. There's quite a bit of push-back on that, but I'm not sure where that will go. Myself I think the obvious and simple answer has been missed: in the presence of List-Post add both a Reply Sender and a Reply List button, and leave the traditional behavior of Reply alone.
Shal
Shal Farley