Probably, the only technical solution not involving redesigning the recipients mail client is to make use of the Reply-To header mechanism.

The Reply-To header is usually used with one address, but in fact multiple addresses scan be specified.

When sending an e-mail to the "reply-all-averse" individual, put yourself and your wife into the Reply-To header.

When the user tries to reply to the e-mail, their mail client should stuff those addresses into the To: field, so unless they edit that, the reply should go to those addresses.

There is the risk that the user's e-mail client doesn't handle multiple addresses in Reply-To (the developers didn't know about this, or else didn't test it).

If this is just for two people, you could create a mail alias that expands to you and your wife's addresses, and use that as the Reply-To target.

what about using an alias that sends to both of you as the Reply-To address?

That's probably the best solution to this problem. I didn't realize that that was a possibility. And actually it doesn't even have to be that complicated. I already have wildcard forwarding set up on one of our domains to send to both of us, so we can just set up an alias in Gmail to send as an address at that domain, then replies will automatically go to both of us. We'll just have to remember to bcc each other when replying.

Good idea! I was thinking about all those techie stuff and workflows via special software ... and then I read your response. Simple and nails the point of OP!

I was reading the question as how to fix people replying all to lists of a few dozen people like school mailing lists, sport clubs etc.

Email yourself and put all the recipients on BCC. That's prevents reply-all, but of course wouldn't prevent somebody from forwarding an email.

I have the opposite problem. I _want_ people to reply-all when I have put someone on cc.

Put a note at the top "Please respond with 'reply all' as those copied are important to this email."

I mis-worded the question, I think. Here's an example of the problem:

On Friday I sent an email to my kid's principal asking about COVID-related things. I cc'd my spouse. When the principal replied they pressed "reply" instead of "reply-all" and so I had to forward their response to my spouse.

This happens with incredible frequency and I'd like to set something up to allow my spouse and I to mutually participate in email conversations without having to train the other people in the conversation.

There’s no etiquette issue in the principle’s email use. Cc’ing your spouse is your way of communication with your spouse. It does not follow from that that your spouse wants to be copied on the reply. I know this from personal experience as the spouse to someone who copies me on email I don’t care about being a part of. Historically, some were school related. Not that I didn’t care, but the executive summary at this end of the day was enough.

Is it generally agreed practice to always use reply all?

You should always use reply all, to keep the discussion together (all recipients in the loop).

The only exception is when a message is broadcast, but the originator is looking for individual replies. Like, for instance, the company office manager asks 300 people whether they are coming to the barbecue. Nobody wants their inbox stuffed with 299 replies.

That might be a good situation for setting the Reply-To header, to save those who habitually use reply-all.

And furthermore if you remove someone it's best to state that in the replay-all-minus-person-X as "removing X"