People and living animals cannot be stored in a subspace pocket because there isn't a breathable atmosphere inside of them. You could feasibly transport people that way, if you take that into account, with the use of air pumps/tanks/scrubbers since some, like Collin and Quinn, live in subspace apartments. (Collin and Quinn do not require air in subspace, but it has not been decided how that one works yet.) Only those like Collin and Quinn could leave a subspace pocket without outside aid however.
There is only one opening to any area in subspace. Subspace pockets require an anchor to access them. Collin and Quinn ARE anchors. The others who access subspace are using technology to anchor a small point of subspace.
Quinn shares the same ability as the Shivae ability of Going Cold, but she is better outfitted to exist within it. When she is in it, she is physically separated from it. She doesn't truly exist there just like they don't. They're just passing through.
Quinn's a dimensional walker and so is Collin, although he does not practice those abilities. Collin tends to linger in the Dream side of the void, which most of them have access to, but the Void is another side to it. Envision it in layers. Everyone visits the Dream on occasion, but you have to die a tragic death to be lost in the Void where you cannot dream. The void is outside the Dream, but it's also a part of it. The only way to be lost in the void is to suffer a sudden death that basically shifts some of the physical body WITH the soul, thereby anchoring it to the real world and not allowing it to pass beyond the Void. While the physical body may leave the physical world, it is still anchored there because it 'belongs' there and only a rare few can do so.
Subspace, is NOT shared with Dream or Void. It has nothing to do with them. It is a specific space that can be accessed from anywhere, but has a definite location that is mapped out with specific coordinates that would be a fourth dimensional coordinate. It is literally like a bunch of lockers with infinite locational possibilities. The dimensions are defined when the pocket is created (and only Wolves have this knowledge) and an item called a 'spacer' is set within the pocket to give it a set size and access for anchor and locking. Anyone might be able to find it, but opening it without the proper keys won't work.
Shivae are more affected by the Void and cannot remain inside of it as long as Quinn can, since she's protected.