Okay, a few points, some of which I'm pretty sure I've addressed before.
(1) You could have a damned large industrial process on a planet, including use of electricity in many and manifold ways, without producing a big enough electromagnetic signature to be easily picked out against the background noise of any star.
(2) Any signature you were radiating could expand only at the speed of light, attenuating the entire way.
(3) Safehold is hundreds of light years beyond the Gbaba's sphere, and the overwhelming evidence at the time of the Gbaba's attack on humanity was that the Gbaba do not aggressively patrol beyond the borders of their own sphere. Rather, they react to incursions into their sphere with the equivalent of a "hot pursuit" response and the extermination of the interloper to be sure he'll never come back. This means that even if Safehold was radiating radio energy out the wahzoo, it would be centuries — quite a few of them, in fact — before any of their radiated energy could be detected by the Gbaba unless the Gbaba happened to be in the area looking for it.
Because of this, the original mission orders for Operation Ark called for a hiatus in which there would be no electromagnetic footprint from Safehold for a long enough period of time for them to be fairly confident that any actively searching Gbaba scout ships had swept through the area and gone home again. There was never any particular concern over what was going to happen after that hiatus was over, because at that time Safehold would be far enough away from any hostile detectors to be beyond threat until it had had plenty of time to rebuild and advance its technology to a "Gbaba-proof" level. (As a "historical" validation of their assumptions, the mission planners could look at the fact that Earth's radio emissions had had ample time to reach well beyond her most distant colony before the Gbaba responded. In other words, they had empirical evidence that the Gbaba hadn't detected them despite radio emissions until they entered the Gbaba's sphere.)
The basis for the Church's prohibition of anything touching on the rakurai is threefold.
First, one of the underlying assumptions of the Proscriptions is that true, large-scale technology is impossible to develop without electricity. Or, put another way, electricity is going to be developed by anybody on his way through to large-scale, advanced technology. Therefore, banning electricity effectively bans large-scale technology and serves as a warning flag that it's being developed.
Second, artificially generated and distributed electricity is something that would be readily detectable from orbit and not something likely to be confused with any natural phenomenon.
And, third, the notion of lightning as sacred and not, under any circumstances, to be profaned by mortal hands provides the permanent remembrance of not just Langhorne's existence but of the consequences of Langhorne's wrath.
Now, at the same time that Langhorne was setting up the prohibition of advanced technology, he was making provision for the "archangels" and "angels" to continue to use very advanced technology to validate their supernatural powers. Merlin's SNARCs are using shielded, stealthed communications arrays designed to hide from hostile tactical sensors — aimed against the Gbaba, at the time the SNARCs were built, but equally effective against any sensors the orbital array might mount — and walking in the footsteps of the technology Langhorne himself ruled was usable by the "archangels." Moreover, there is a quantum leap between the initial production of electricity and the highly advanced applications of it inherent in Merlin's toys. In effect, it would be relatively simple to build a protocol into the bombardment platform's sensors (assuming, of course, that the sensors are actually prepared to blast technology on the planetary surface in the first place) which differentiates between "technology so advanced it must be being used by the 'archangels'" and "technology so crude that it couldn't be being used by the 'archangels.'" If it's the latter, it needs to be smitten . . . quickly; if it's the former, leave it alone because it's being used on Langhorne's business.
In that respect, Merlin is hiding in the shadow of the original archangels. Although that, of course, assumes that all of his carefully stealthed technology is detectable in the first place. He has no intention of giving away any more detectable signature than he can avoid, of course, but from the perspective of the reader that "shadow" should always be borne in mind
As for Nimue's Cave, it's buried beneath 7.5 miles of solid rock and iron ore. During the period before her PICA woke up, the entire installation was, indeed, at absolutely minimal levels of power, and its primary power source when it came back online is a geothermal tap, so there's no betraying emission of burned hydrocarbons or neutrinos to give it away. Assuming that it was possible for Nimue to excavate a similar underground complex and install a major generating system there, and find the labor force to work in that complex without popping flags with its members about violations of the Proscriptions, then, yes, Charis could have electricity for applications which require it. Of course, digging something that size in anything less than several decades without using technology which in its self would probably trigger the arrays (assuming that they are triggerable). And, equally of course, they might find it a bit difficult to explain to the rest of their workforce where the products being produced using that electricity were coming from and how they were being produced.
The bottom line is that no one in Charis really knows whether or not generated and distributed electricity would trigger a bombardment. They suspect that it would, and in this instance they prefer to err on the side of safety, but they certainly don't know it. On the other hand, they do know that "profaning the rakurai" would represent a crossing of the Rubicon. It would be an open, explicit defiance of the Proscriptions which no one could argue away. That is something they cannot afford at this time or, indeed, at any time until after the proscriptions themselves have been successfully invalidated/overturned following the "reveal" of the truth about Langhorne and the archangels.
So while there are what you might call "technical issues" bound up in the uncertainty of how the bombardment platform would respond, there is also an overriding reason — found in the Proscriptions — why electricity is impossible. In the meantime, the electronic footprint of Merlin's stealthed recon skimmer, the SNARCs singular transmissions, the personal coms of the inner circle, etc., are extraordinarily difficult to detect in the first place and (apparently
) trapped in the filters that were established to allow the "archangels" to employ technology.
Don't know if all of that really makes it any clearer, but there it is.
