This is better solved by an add-on script like this one: Select Code Block Buttons Script.
That app currently only supports 'select all', but could easily be expanded to include the other features you requested.
That said, here's why I think this shouldn't be implemented for everyone:
- Including too much code
The ideal SO post is a good explanation of the question/answer with a minimal code snippet to illustrate it. Adding extra support for huge walls of text suggests to users that lots of code should be included. Of course, in the real world it's sometimes necessary to include significant amounts of code, but we don't want to encourage that.
- Cut and paste instead of learning
Ideally people come away from SO with a new understanding of whatever they had a question about, but there's definitely a risk that they just come away with some code that solves their problem but that they don't really understand. There are already repositories for useful snippets of code out on the web, we don't want to inadvertently turn SO into one of those.
For all that, I think that the strongest argument in favor of such a feature is better enabling SSCCEs in questions. Lots of modern software doesn't lend itself to a SSCCE because even 'Hello World' requires multiple files in a particular folder hierarchy. Anything that makes it easier for answerers to quickly test questioners' code to try and help is good. However, if you're going to be firing up a small project to check out a problem, you really want an online emulator like jsfiddle or a public repo/bundle with everything already organized correctly instead of having to cut and paste a bunch of text, even if it is easy to select.