I also think that Menchi could be a Solarian survivor.
You are amazing. I think I'll capture this one and make it my new pet theory (pun intended).
Huh, the cyborg theory is certainly plausible, given that Excel can go toe-to-toe with Iwata and Ropponmatsu. Of course, if they are cyborgs in the usual Excel Saga sense, I think Elgala probably wouldn't have noticed any difference in "President" Excel's weight after the switch. Perhaps they are more refined than Shioji's robots though...
I'm inclined to think (with little to no proof) that Elgala, Hyatt, and Excel (and perhaps even the Ropponmatsus) are all facets of the Solarian Princess's personality/spirit/whathaveyou imprinted upon regular citizens Il Pala has "recruited". It's been hinted lately (volume 19, I think?) that Il Palazzo possesses some sort of over-technology that brain-washes and/or inspires loyalty, though whether that was Kabapu being flippant or not is debatable... I suspect Il Palazzo subjected himself to this over-tech as well so that he could inhabit a corporeal body after whatever destroyed Solaria presumably killed him and the princess (though how did Kabapu survive then?).
The holograph tech could be him asserting himself incorporeally, and the "donor" body could be why he has his split personality issues. This could also be why Excel has her uncanny biological expertise, why she's afraid of needles (painful experimentation and the trauma of being reborn) and why the ACROSS girls cannot remember their pasts. Hyatt's condition, in this case, could also be explained by both "inferior genetic substrate" and problems with the "brainwashing" overtech.
I suspect the core is what these facets were extracted from (probably by Tenmangu), and it is the key to the reincarnations of both Il Pala(?) and certainly the princess.
This could also account for Tenmangu's disappearance-I think Miwa was either his first experiment with imprinting, or that something happened to her and he was forced to resort to the overtech (or both). With Kabapu knowing he abused the overtech, Tenmangu became android!Miwa, hiding himself in plain sight, where he was then free (in the guise of caring mother) to toy with Lord Il in addition to Kabapu.
Though not quite logical continuation, I suspect the real Miwa is Momochi (who fits Miwa's pre-crazy personality type), probably in android form, who both spies on and assists Kabapu as neccessary for Tenmangu.
...
Yeah...
So basically, a lot of my theories are Micki!'s modified.
Great minds think alike, no?