Please don't: when used for code, preserving lines (even when requiring scrolling on mobile) is a good thing. 100% width is fine, no reason it needs to be as narrow as it is.

If a blockquote format is desired, that's great, but don't change the code formatting to make it a slightly-less bad (but still horrible, because of monospacing) way of presenting blockquotes.

How about a few profile fields where users can customize some aspects of the HN CSS that is used just for them when they are logged in.

You know, like how there is "topcolor" already; just a bit more of the same.

Yes.

And on copy/paste, the linebreaks in the original are preserved, which obviates the code objection answering you below.