@Elliptica @ceo_of_monoeye_dating @roboneko @mushroom_soup @Inginsub @cassidyclown @Kirino I am totally ignorant of the topic, but here are some thoughts:

Isn't a wormhole supposed to be kind of like a black hole? Or rather a black hole could be one end of a wormhole? Naively I think that your momentum changes as you fall into a black hole. So there would be no reason why going through a portal should conserve momentum (in the "external world's" frame).

It seems to make sense to say that "something is in the hole" (in the event horizon?) but not that "the hole is in something" because a thing lives in spacetime while the hole is a distortion of spacetime. Whereas a portal lives "in" a portal wall.

Even in a special relativity setting, if you jump into a portal, it seems unclear when the other portal should spit you out. In a different reference frame, wouldn't you appear to emerge from one portal before you step into the other? I wonder if you could create clones of one of the portals by passing it into the other portal while in some moving reference frame. (spoiler alert - isn't this like the plot of Primer?)

Going along with your portal-powered ship, I was thinking about using a portal to do free work like this: take a strong rod, and move one end of the portal halfway across the rod. That half of the rod emerges out of the other end. Now tilt the first end of the portal back and forth. The other half of the rod starts to swing back and forth. If a portal can move "for free" (just like how the trolley doesn't feel the 'impact' of the tied guys), then we can do work by having the rod push on something with resistance.