How was that supposed to happen?
Wikipedia makes this clear.
David leads Calvin into his pod and launches into space as Miranda launches hers. One of the pods hits debris and is knocked off course. Calvin then lunges at David and attacks him as he struggles to send his pod into deep space, The earthbound pod performs a controlled re-entry ...
There was only one Calvin.
After killing the commander, Calvin got back in to the station through one of the thrusters, IIRC, after they had burned all the fuel it had trying to keep him out.
As for how he got into Hugh's leg, basically, Hugh had some kind of sympathy for the creature and allowed it to go in there, but Calvin was already definitively back ...
We don't know whether Calvin is actually "comfortable" outside, or desperately fighting for its life on quickly depleting oxygen reserves - the latter could be an in-universe explanation.
But realistically, you're right, it does not make much sense. If Calvin can survive for several minutes without any atmosphere at all, then a slight drop in pressure or ...
They are still on the ISS. The second image is an external view of the ISS as though from the point of view of (the movie equivalent of) an omniscient narrator; it is not what David sees. Presumably, the filmmakers chose to show the ISS from this angle because it gave a clearer or more complete view than what David was seeing.
When an object (in this case a capsule) falls to Earth, it gets slowed down by many different things:
First, the Earth's atmosphere will dramatically slow down the object. While this is easy and free (does not cost anything), it comes with the risk of overheating (which is why capsules have heat shields for reentry).
When the object has slowed down ...
The oxygen torch is creating energy, heat, light, and oxygen. We do not know for sure which of these things Calvin actually needs, but we can assume he is holding the torch to absorb as much of it as possible.
Despite not having found much information about a similar looking Oxygen Torch, I did find this, a fuel tank for an oxygen torch.
I assume that an oxygen torch in the movie is just a different-looking kind of blow-torch that uses oxygen as fuel. It matches all the 3 features you described.
It has a compressed oxygen tank as a supply, thus is able to ...
Hugh has some strange, inexplicable (read : stupid) bond with Calvin from the beginning. He's the only one calling it Calvin, like it's a person. He feels that the world's stem cell research (and possibly healing paraplegic folks like himself) would take a leap ahead because of Calvin.
Calvin has other plans. It wants to consume them. After Calvin enters ...