Thanks, robtillaart!
As far as the Highlighter being in the forum (I think you meant
arduino.cc/forum, right?) - I can't say I'm very familiar with the platform it runs on - Simple Machines - even though I worked on its precursor before. I'll definitely take a look at it and see what I can do. But, obviously, the decision to use it or not should be the administrators' and I'm not even so sure who they are and whom should I contact if such integration is successful.
As far as the WP plugin itself: I would have to admit, it would be much easier to run
Ctl-T in Arduino IDE before copying the code and pasting it into WordPress rather than having WP second-guess the formatting. In addition to this, it may be beneficial to leave the final formatting to the programmer: for example, I myself prefer to reset indentations (that is, move the text back to the beginning of the line) where I have several (4-5 or more) stacked indentations. Anyhow, I think it will be more flexible if the actual formatting of the text is done in the IDE.
a "strange " indentation
- do you mean, like on Line 112? Well, that's because until this morning I did not know
Ctl-T command existed!

I guess I got a bit spoiled with auto-formatting on the fly in other editors I use, so I thought Aruino IDE is just not auto-formatting. Didn't look like a big deal to me either, but now I know I was wrong! I should have done
Ctl-T before copy/pasting the code into the post.
Anyway, if I find a way to do it in SMF forum software, I'll post here and then admins will decide if it's something worth using.