ok.
1. the multiverse having one bigger timeline is non sense, you can't have a multiverse (composed of 12 universe, or 13, or whatever the number), and then have a bigger timeline, and no explanation for this bigger timeline that we are assuming it exists.
you either have a multiverse, which each universe having a individual timeline, or you have a multiverse, with all of its space and time being affected by the timeline, but you can't have a multiverse and then have a bigger timeline, without having any statements for the later.
2. the neutral space is just, a neutral space, is not a bigger dimensional structure, it is just a way to connect the universes, that does not make it a higher dimensional structure, is like having a big solar system with 12 planets, and then the solar system is covered by a asteroid belt.
3. the time chamber is just a place with a different time flow, it is not explained to be another time axis, the explanation for it is that it is a place where time flows different from earth (or the normal universe)not like a different time axis or something like that.