Paddock Club is the area (BIG area) where teams and sponsors receive the VVIP with restaurant service etc, I’ve heard that teams have to pay to Bernie something around 3000 € per ticket but don’t quote me on that since I’m not sure. Anyway for us mere mortals it’s an off limits area... and paddock club pitlane walk... well I can just assume it's a tour in the pitlane for the VVIP guests.
Last time I was at the track for the Friday practice it was a couple of years ago and the tickets were in sale also at the track (35€ for the grandstands IIRC) but since these are numbered tickets I assume that it’s better if you get it as soon as you can, my best suggestion is to look for the official website of the Spa track, surely you will find info there.
As for Thursday events, I really don’t know (let me know when you discover it
), a couple of times I made a “clandestine tour” around the paddock etc on Wednesday but on Thursday I never tried. Actually the whole city is a mess during the GP weekend (population on normal days 130k, on GP weekends close to twice) and I often prefer to stay at home with maybe just a little tour downtown on Saturday morning, I’m not really agoraphobic, but too much is too much...
Hope this helps