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Is it possible that these are being used to influence prominent individuals from other factions without their knowledge ?

Like say, is it possible that a High-Lord of Terra, (like say, Goge Vandire), or a Tau Ethereal, or an Eldar farseer ,or a Tyranid organism like a Norn-Queen or Hive Tyrant could be under the active influence of one without anyone ever finding out ?

Since the Necrons are patient and manipulative it seems like they could use it as a cheap way of enhancing their goals over the millennium without anyone being the wiser.

Also is there any effective range on a Scarab and can they subvert machines like Wraith constructs or augmented individuals like Dreadnoughts or a martian tech-priests ?

Mind-shackle Scarabs have to be in physical contact to work. Presumably the ones used in battle get ripped off in short order because they're attached over the skin, but in Faith and Fire there was a Sister who had one implanted, Goauld-style. Of course, being a Sister, her strength of will was powerful enough to short the thing out and make it glitch all to heck (she presumably passed a lot of 3d6 leadership tests!)...

Basically, though, it seems they need to be in physical skin-to-shell contact, probably getting their little claws literally into your nervous system to have any effect. I suspect that anyone important enough to subvert this way who is competent enough for it to be worth it would be able to fight off the scarab's influence eventually.

Though there was that one guy at a party held by the planetary governor who literally exploded with them, and through those means the Necrons took the entire planet.

?

There a short snippet in the Necron Codex which mentions something like this, though in an off-handed manner. An awakening Tomb World is testing the strength of its legions and their efficiency. In order to do this they have various moderately powerful / influential.. erm... 'aliens' (aliens to the Necrons) unknowingly implanted with Mind Shackle Scarabs, and are having these individuals fake results / tell stories to lure armies to their world which they can decimate and test their forces on.

Near the end it explicitly mentions them 'activating' the scarabs that have infested a Tech Priest, and that they're having him fake results to lure a party of Ad Mech to their world which they can test their guys on.

Actually, provided he has contact with Necrons at some point, MSS would certainly explain Abaddons failed crusades over the years (I don't care that it was ret conned, he failed every bloody time)