"I'm going to move this enemy with my ability, but there are two more enemies behind him, so he gets flung 7 squares away from where he was."
As far as I am aware (take with a grain of salt unless someone can point to confirming / contradicting official rules), moving and placing are different mechanics with respect to what happens when there isn't an available place to go. When "placing" (such as reinforcing, special quest rules, or for some other self-movement abilities) then you follow the "place in nearest empty space" rule if you can't place them in the target one. Most abilities that change where an enemy figure is use the word "move". Moves DO NOT follow the "place in nearest empty space" rule, so if there is no valid place to be moved to, you just don't get to move them.
The web comment is a reference to the rule that if you get webbed during your movement (i.e. via the web trap card) even though you are technically immobilized you still get to finish your movement as normal; which does not make any sense and seriously reduces the effectiveness of the card.
The web comment refers to using the Web Trap to immobilize. Immoblize says the hero may not get movement points, but it does not end the current move action, so a character like Silhouette who is webbed in the first step of her 10 move heroic still gets to finish the other 9 moves and pick up all treasures she passes, as per her Heroic. Then the immoblize takes affect.
This is not QUITE correct. You have to understand that there is a difference between Move Actions and other movement. According to the FAQ, if you are immobilized during a normal Move Action, you immediately stop moving and lose all of your remaining movement points. If you are using some other method that allows you to move that is NOT a move action (such as Silhouette's Heroic Feat, as you mention, or basically any other form of movement that Immobilize doesn't restrict, which are relatively rare), then that rule applies and you can finish your movement because Immobilize wouldn't have prevented it in the first place.
Also, if you feel this is too overpowered, it's a pretty simple house rule to change Immobilized to also include ALL movement, and one that PROBABLY wouldn't be too game-breaking. Just make sure you discuss it with the rest of the group beforehand.
The problem with Silhouette's heroic feat is the fact she get's free search actions. That can allow her to get 4-5 actions in one turn. It's very unbalanced.