As temporary faculty, how to deal with a colleague who, via email, questions agreements made in a meeting?

Actually, I would ignore it. It is fairly typical behavior among some faculty. It might not even be abuse, but just the other person having time to reflect after the meeting. If the comments made were on-topic and not a personal attack, then you have nothing to gain in your current position by doing anything beyond arguing for your position. It could even be that the other person is uncomfortable in meeting generally.

I once found that offering suggestions for improvement of policy, as a new faculty member, is sometimes definitely not appreciated. I suffered setbacks because I suggested that "the way we do things here" was counterproductive. People don't like to hear that from a new person or an underling.

But yielding graciously, even if you don't feel it, could put you in a better position for future employment there.

On the other hand, you are justified in responding to a personal attack, but do so through "channels".

An earlier comment was removed which said that such a head-over-heels turn would deserve a PIP in industry is far over the top. I do not know why this comment was removed, I thought it was perfectly apt. Yea, it is not nice when somebody tries to underhandedly and one-sidedly modify an agreement decided publicly, but it is far from an actionable offence.

As response, it is perfectly sufficient to say that if they disagree with the outcome of the meeting and want to change it, and OP is unhappy about this proposed change, it would need renegotiation.

So, an answer could be: "In our meeting, we agreed to which seemed to be all right with all participants; this included me - and, given that you did not express anything to the contrary, also you. If you wish to modify this outcome now, this would need to be coordinated between all parties; in this case, I suggest you call in another meeting of all the parties to realign the duties."

I very much doubt that that person would dare to do that, given that the others will ask themselves why they have to waste their time again on a question which already had been decided and closed.

I suggest checking with the department chair to find out what he wants you to do --- continue with your part of the work as agreed during the meeting? trade assignments with the recently tenured person so that he can develop that "clear vision"? do nothing and let the newly tenured person do your part of the work in addition to his own?