In the core rulebook, it's written that the current state of Empire has weakened the Imperium.

But why?

The Imperium is able to destroy an empire like Solomon in few hours!

Maybe they need to control humanity to gain the power to destroy Gaira and C'iel.

The Imperium is lead by powerful individuals, who don't care to conquer Gaia, and they aren't very interesting to Gaira and C'iel. (Or maybe they are no more)

Their goal is control the humanity, maybe because if all humans are synchronized with what they want, Gaira and C'iel will be weakened.

But every failure could reveal their plan to Gaira and C'iel.

After Rha's "incident" Imperium had to sign a sort of treaty of non-intervention with other Powers in The Shadows AND Shajads & Berils (on behalf of C'iel and Gaira). The Imperium cannot simply go and destroy the Empire, because probably that would go against what they're allowed to do by the other powers. Imperium's aim is destroying Gaira and C'iel (with their Shajads/Berils), but it's pretty obvious they still don't have the power to do so, hence they have to be vary cautious in order to avoid the gods (and/or other Powers in The Shadow) understand their plans.

In that chapter it's specified that the 3 powers in the shadows are keeping each other in check, plus they signed the Pact with a specific clause to avoid interference with the emperor's descendants.

Basically, they can't go up against each other because they are nearly on the same power level, and they can't act against 'humanity' (or try to control it) because of the pact.

So the Imperium can't do anything against the Empire (and so making a big change in human history) because that would break the Pact, and they can't do nothing to stop Lucanor from doing his research since they accepted the clause.

The 3 powers aren't likely to go all 3 together VS gods&demigods, since each of them has a different goal to achieve.

It's just a very big typical mexican standoff (like those in western movies). 3 guys pointing a gun at each other. One move means death.

But it's not true the Imperium did nothing: there's a hint about the whole messiah thing in the same chapter.

If a Game Director decides to give credit to that part, then it's pretty obvious that everything IS actually going as Imperium wants. But still they can't act directly.

More than a point to develop, is a system to keep the settings in check, making a move on that level would practically end the game and start a new setting.

A few things are going ok to the Imperium...a few aren't...

For example they've "lost sight" once and again of the 30 black pieces...a technology that can possibly allow humanity to "equal" them (actually they attacked Solomon when the Senators were planning to use the technology to turn themselves into protodeus).

Also, they've designed a few (20 I believe) weapons that are able to absorb the gnosis and powers of those who they kill...the weapons were dintributed all around the world and wielders of those will fill also eventually compelled to fight each other, so that one of the weapons will absorb the other. The final product of this thing should be some sort of "ABSOLUTE PERFECT WEAPON" capable of killing Shajads/Berils. BUT the guy who killed the Messiah was wielding one of those weapons...he was a descendant of Andromalius and his nanomachines interferred with the enormous Gnosis that was "sucked up" by the weapon, turning him into Omega. The so called entity fled to an existential Gap out of Imperium's reach, bringing along himself the weapon (kinda Longinus lance). That's a big bad problem for the Imperium that has almost lost his investment. Actually it seems it was Millenium who handled this weapon to the unfortunate "Longinus" (his actual name has long been forgotten), probably because she knew that would be a pain for them. She seems to be interested in messing with Imperium...she helped both Rah and Eljared, after all...

In my opinion, more flaws the Imperium has, better it is, because on the contrary it seems the master's police:

"Are you trying to change the setting without my consensus? Now I call Imperium so they crush your PC!"

And I read again the story, Solomon wasn't destroyed in few hours: in fact, later Romulo has killed Abel, and Romulo was a citizen of Solomon.

Probably, in few hours the Imperium has destroyed or stealth the technologies of the empire of Solomon, and probably killed their most eminent scientists.

And why Barnabas has changed his orders about Elisabetta? Maybe has he noticed the interest for her by Gaira and C'iel?

And who is Barnabas? The first human met by Gaira and C'iel?