I've read your whole text several times, but I did not get it yet. I guess the spider-thing (looks very evil btw ) is not supposed to pass through solid matter? Please try to explain the problem in more detail!
I think it's best to give the spider no world collision detection at all, just check for the player model (realizable via group or push).

The level is nice as usual (downloading it right now), there are only some minor issues:

• In the 4th picture there's a wooden path (?) in the background. It looks quite weird, sorry. I would either enhance or remove it. (EDIT: Looks okay in-game!)
• The whole staircase (21.jpg) looks awesome, I really like it! (Not really an issue! :> )
• EDIT: On the right staircase you have to pass through wooden planks to get to the next level.
• 23.jpg: The wooden plank in the middle of the screen is floating in the air. The big circle with the dark grid could use some work, there are visible seams next to the boy.
  I would add some kind of salient edge to the brick ground (not only the circle) where it meets the dark grid, so both elements blend together nicely. Alternatively, you could make the circle smoother. (EDIT: It's funny to run below the spider and to see the player through the "dark parts" (don't know the right word).)
• What's the purpose of the telescopes? (EDIT: Okay, now I know it! :>) I would add three small drop shadows at their feet.

Keep on doing such great levels!

EDIT: Do you use (sometimes) level geometry (WED blocks) for your levels?