You're correct that there is no criminal penalty for using EAS tones per se, but if it was me I think I'd be inclined to make very sure that I'm not using anything that actually decodes to a real alert. If you're interested in avoiding that entirely, take a look at the old EBS alerts (the ones I grew up with in the '80's and early '90's), as they aren't going to have any risk of triggering modern systems. (And those of us that are older have an instinctive response to those dissonant tones....)
A few things to keep in mind - alerts are intended to be short and easily digestible. It's unlikely that any sort of news about what has happened beyond the general nature of the threat would be communicated, particularly from beyond the borders of the US. So it would be more realistic (and, if you phrase things right, possibly more suspenseful) to be vague. If there is a need to give more information, generally the expectation is that the listener would be given instructions on where to tune for that (which then gives you the opportunity to mock up a network failure for that). Something like 'We do not know' would almost certainly not be included in a warning (since when has the US government ever admitted to not knowing something?). Nor would 'The world has ended' - more likely, a 'shelter in place and await further instructions' would be repeated indefinitely. Can't have people panicking in the streets.
Check the list of issuing authorities available in the EAS codes. I doubt the military would be issuing anything within the borders of the US (they're pretty limited in what they can do domestically for historical reasons); FEMA or the Office of the President would be more likely issuing authorities, if it's not just going to be listed as the EAS. (Or, for fun, use the National Weather Service - they're on top of everything.)
The network for issuing the alerts depends on injection of the message at key stations, with secondary stations monitoring those to pick up and pass along that information (or at least it used to). Pauses, dead air, snippets from accidentally open mics as network feeds switch, and abrupt transitions are entirely likely (and again will help build suspense).
Proofread and spellcheck - the Federal government may be awful about keeping their population (mis)informed, but they sure will format it nicely. 'Emergent' is almost never used in place of 'Emergency' in the US, at least that I've seen in my experience being startled by system tests, tornado warnings, and the occasional Amber alert.
I did enjoy these - this sort of horror/suspense is fun once in a while!