I really like the writing for Allirea, but it's incredibly frustrating having important plot details constantly being hidden behind a power curtain.

There's actually nothing that would be that revelatory to the reader in Siobhan and co.'s dialogue which I've hidden behind all of Elspeth's zoning out. Like Elspeth flinging summaries every which way as she goes, it was a way to gloss over repetitive information dispensing. You guys already know who Allirea is and what she does and what her motives are.

Could Allirea use her phasing as a signalling mechanism? For instance, someone she'd met could reasonably often check if they can remember her existence (if doing it mentally is prohibited by the power, then have a PDA or something that sets off an alarm with the word 'Allirea' every five minutes) and if so, take some action.

This is plausible in principle.

The relevant question here is, I suppose, whether Allirea's phasing immediately affects everyone on earth who knows her, or not. If the former, is there a lightspeed delay?)

I'm not planning to figure in a lightspeed delay on account of that being a pain in the neck. It propagates immediately (or at least over the course of the time it takes her to (un)fade). And it's "fading". "Phasing" is what wolves do when they shapeshift.