I wonder: What is the best way for asking for support? Apparently there's no use reporting them in my account -> my e-mails, as these are routinely being ignored (of my four e-mails, one got answered in two days, one in ten days, one in 24 days and one (posted on March 17) hasn't been answered at all :-(

What do you need support for?
If it's a problem with a specific game, try posting in the forum dedicated to that game.

At the moment, a problem with the site - the non-working RSS due to invalid structure (duplicate GUIDs).

Problems with site functionality won't necessarily get a response at all, they have been known to just fix it and leave it at that. However, if you want to notify them of an issue with the site, you just need to use the appropriate contact form in the support section:
http://www.gog.com/en/support/contact/general_website_questions

They dont answer on weekends and staff is relatively small so it's possible some will get lost in the shuffle. Keep bugging.