George Lucas threw the EU/RPG/Comics "canon" out the window with The Phantom Menace. He completely contradicted the way the Clone Wars happened.
I have no idea how the video games are supposed to be worked into the canon aside from being tacked on, so I can't comment (other than commenting that I can't comment
).
Not really. Almost nothing about the Clone Wars was known before the PT came out. We knew of a few characters who fought in the war, a few ship, vehicle, and weapon models used in the war, and where a few battles were fought that's it. Palleon remembered fighting Clones during the war but that was covered by a comic where the CIS tried to make its own clone army and they went insane. It wasn't until TCW that major problems appeared when they decided to make the Mandalorians neutral when the ROTJ novelization stated that they were fighting the Republic during the war.
Yes, though what Zahn wrote was what was considered "canon" by the books and RPGs. A lot of fans (I'll admit, me included!) assumed that this was real canon.
GL ignored it and did what he wanted.
It was real canon. But Lucas has always maintained that his ideas come first and foremost. But to be fair to Lucas, it's hard to know if he ignored it. There's no way of knowing just how much he knew about what other authors were saying about the Clone Wars before he penned the PT.