Make it easy to read emails either in plaintext or html mode (by default / always / once).

Yes it should.

The solution of mutt i configured for me on my linux is - for me - much better: no html, only plain text. And if there is no plain text section, the HTML is parsed and converted to plain text. All links e.g. are converted to plain URL. The best advantage of it: you see phising mail immediately. At the moment it is impossible to recognize it (today e.g. i got a mail from "amazon.de" - of course it was not amazon. But Sailfish Mail Client says it was from Amazon. And it was not possible to see the real sender or even the plain link).

What you described is the standard kmail behaviour: no html, just plaintext. As you said, this form has many advantages. But these emails can sometimes be hard to read. So kmail allows the user via a simple click to temporary interpret the mail as html. If Jolla would implement it in this way. A user like you wouldn't have to do anything. If a user just sometimes want to see html, he simply can click a button once and the mail gets rendered. If the user want a simple to use mail client with eye-candy mails and not so much security he can switch on html rendering for all emails. So I still think the kmail solution would be best for the majority of the users, but of course your proposal still sounds a lot better than the current Jolla default rendering-everything-in-html.

I would add it the "setup account" wizard i.e "Would you like to use fancy formatting for emails?" type question.

this way, those that want/need to have lots of colours and bling only see that, those that select "TXT only" identify themselves as being more technical, so able to cope with options ;)