I tend to go back to the fundmental mechanic when faced with dilemma's like these. Specific Beats General is a fundamental. However, in the absence of a specific, you MUST rely on general.
In general, A creature/character performs all actions on its turn. There are specific ways to trump this (immediate interrupt, e.g.).
So, for example, a wizard creates a zone on his turn in an area including a creature. The zone now "exists" on the wizards turn. The text SPECIFIES damage on entering into or starting a turn in the zone. Therefore, the creature that has not moved, and whose turn it is NOT takes no damage since it did not enter the zone (the creature would have had to act or forced into action - in another words, an action must be spent by somebody to cause the creature to act. Passively standing there while a zone moves or is created over you is not an action by the creature or forced onto the creature. The zone "acted", not the creature.). The wizard then spends an Action Point and Thunderwaves another creature into the zone. The specific rule specifies entering a zone, so the creature takes damage.
Next round its the creatures turn. there are 2 in the zone, both take damage, then move out. Another creature charges the wizard, but enters the zone on its way. It also takes damage.
Note how the bulk of the damage dealt is happening on the creatures turn...this is the General rule in action. The one pushed creature got an extra dose of damage on the wizards turn. This is the example of the specific beating the general rule on action management. Forced movement is still movement.
If you need fluff to explain while the guy standing there took "one" hit on his turn, while the forced guy took "two" hits, remember that a full round is the "thing" while the initiative order is a game device we use to organize and manage it. The creature standing there sees the zone start to form and high tails it out of there, catching some damage as it formed around him as he left. The guy who was pushed into it felt the damage as it formed as he was pushed into it, and took damage when he regained control and moved out. All of that happened in a "round" and it all happened in real time, not in discrete initiative order.
As for the zig-zag and the save. I don't agree with the save unless the power specifies it is terrain that triggers a save. However, I also don't agree with multiple damage from a zig-zag. A single action caused the "entering" to occur. Zig-zagging doesn't cause you to enter multiple times on that action. You arte considered "entered" for that action. However, that said, If I pushed a creature in, another character pulled him out and a third character pushed him back in, etc. then I would consider it multiple entries.
my 2cp