Questions - Why are certain things excluded?
Maybe, but our focus within the project is humanity's expansion. You are welcome to submit your ideas, like any other, for review by the group and potential inclusion into the setting. Before pursuing an alien species you may want to look at the options available through tweaks, splices, provolves, and genetically engineered life forms.
Not at all. In fact, it should foster diversity, because it forces us, as the creators, to be more biologically innovative. Not being allowed to take the easy way actually increases the creativity. As a hard science setting, we feel the universe is far too wild and amazing a place to have something as tame as the prototypical humanoid as the norm for sophont lifeforms.
While it is true that new discoveries are always turning up, in physics and astronomy for example, this does not give us license to invent hundreds of unknown fields and particles and radiations. New hypotheses and discoveries still have to confirmed or refuted, and this may take years or decades of observation and experimentation. Often the apparently new phenomenon is a false alarm or turns out to be just a variation on an existing phenomenon. For this reason, we would rather err on the side of caution, and only incorporate into the setting that which is known to exist, is not logically impossible, and does not break the laws of physics as we currently understand them.
It's plausible at least, that the universe is layered. Although the existence of other dimensions is necessary for some physics theories, these dimensions are not like the standard SF trope of "hyperspace" or "alternate realities". Rather, they are sub-microscopic (planck size) in scale, and only play a limited role in the setting, as tools used by the most powerful intellects of archai level or higher. You could look at this as having more dimensions rather than 'other' dimensions (i.e. 8-dimensional space-time representations, but not two separate, parallel 4-dimensional ones).
Unfortunately, there is no evidence of anything credible that points to the ability to generate gravitons/bend space-time aside from increasing the mass of an object (i.e. natural gravity). The makers of popular SF for movies and television find it understandably inconvenient to keep all of their actors suspended on strings. This has had the unfortunate side effect of encouraging viewers to assume that we will easily invent artificial gravity eventually, despite the fact that the laws of physics dictate otherwise. However, this setting does have artificial gravity generated by centrifugal rotation. Apart from that the only way to generate gravity is to use mass. You can compact the mass into a very small space with OA tech, but you can't simply dispense with it. Because of this OA does have technologies that can manipulate gravitational effects to a limited degree. However, these technologies are all based on the manipulation of very large masses compressed to very high densities and often very small sizes.
Because mental mind control, radio-quality telepathy, telekinetics, and the various other 'psionic' powers have not been proven so far in real life in any replicable manner as generally defined by the scientific method or community, so they are not included. This does not mean that we are dogmatically saying they are untrue, mind you. We take no definitive stance either way on this matter. Such things may exist, but in the OA setting as in the present world, they are more likely to be rumor and urban legend.
See this page
for a discussion of this topic.