Zero fast. There is no energy being transferred to the people, they would plop out and push into each other as they are forced through.
If you blocked the stationary portal then the portal moving would essentially just be a wall, no one would go though.
This whole relative thing makes no sense, energy isn't just created because it's observed by someone else, the door is moving not the people so them sitting there won't suddenly be catapulted going through a moving portal, where is that energy created?
Your wind question is confusing.
Yeah they kinda lose ground under them as the portal goes through them, they aren't pushed, they are moved through space. No additional force or momentum is ever applied.
In the game you aren't pushed through ever, you walk through or fall through them. You appear at the other side of the door.