This... This could work.
I tend to overwork things, looking too far into the future, but your stable idea could save us three magnitudes on the wand.
We couldn't use a Group target, sadly, not unless we want precise control. Not that this changes much*. This could work for unleashing all our servants on a given aggressor, though.
And I'm loathe to put a 7-years expiry. For such a project, this is a waste of time, in my opinion.
70 years is more like it. Hopefully, either we or our successors will be able to do something better, and, at worst, we'll leave a laboratory text behind.
Sigh... This is a little far from my plans. I wanted eternal guardians, but, sadly, I am not yet up to the task. This will have to do, I figure.
Wand: Base 5, +2 voice, +1 concentration, +0 ind = level 20, and then +6 for 50 uses a day, +5 to maintain concentration, +3 for only allowing people attuned to the aegis to use it, +8 for 16 penetration = 42.
With a lab total of around 63, this makes 21 more points, which becomes 42 with a 70 years expiry... JUST!!! She can to it as a lesser item.
The room will be more difficult, though, since she can't count on spell familiarity to boost her lab total. So she'll either need some help, or do it as a greater one.
This is another possibility. Knowing the spell "Voices from Hollow Spaces", I can bind ghosts to circlets we'd put around each elemental's head, and even instill that in an item, easily. In fact, I had ideas around this
But then, unless dealing with amicable ghosts, like former covenant members, we'd need a way to compel the ghosts, and a means to give them control over the elementals. All of these are certainly possible, but, as far as I can see, only on a case-by-case basis, meaning we'd have to enchant each and every circlet. Unless I'm missing something, of course...