Can a manifester that has separated body and spirit, such as by the power Stealsteel or by some other means, still return to its body if the manifester's body is in stasis, such as by the power Quintessence or by some other method of stasis?
The books certainly aren’t going to define this.
But it seems to me that a spirit, once immersed fully within quintessence, would also be subject to the time-freezing effect. That means the spirit attempts to return to the body, gets into the quintessence, and freezes there, unable to complete the journey. When and if the quintessence is removed, the spirit continues and reunites with its body just as though there had been no quintessence in the first place and no time had passed—which is precisely what the experience is like for the spirit.
If the “returning” is instantaneous, then the creature is still frozen within the quintessence. There is a quasi-philosophical/semantic argument about whether or not the spirit is now combined within the body, and whether that means something about an object frozen in quintessence has changed (which should be impossible), and under the game rules there may be significance to this if so, but we do not have nearly enough information to go on to really come down on one side or the other here. I would probably argue something like “even if it’s instantaneous, any externally-measurable effect of the rejoining does not happen instantaneously and so the body still, to an outside observer, appears as it did before, without its spirit.”
I would limit this answer, if it weren’t already obvious, by pointing out that quintessence has a lot of unknowns with it, and without knowing exactly what effect we’re talking about here, it’s really hard to say much of anything.