Just like there were two Picards when he came back with Kirk?
The evidence could be found in the missile site on Veridian III and the facilities on the Armagosa Observatory. Picard knows Soran's plan, and a torpedo armed with trilithium could be found there. He could have the Klingons ambushed before they killed the Romulans and question both groups for information about the source of the trilithium.
We have no proof of there even being a torpedo launcher on Veridian III beforehand.
It didn't look like a new installation. And even if it was, there was still Soran's secret lab on the Amargosa Observatory with those handy trilithium-tipped torpedoes.
If Picard were to exit early enough to send fair warning to Robert and Rene, he'd have plenty of time to also be in the vicinity of Amargosa
before the
Romulans attacked. Soran, the Romulans, and eventually the Klingons would all be there. And while Soran and the Klingons wouldn't likely cooperate in an investigation, the Romulans only wanted their trilithium back and might work with the Starfleet crew to gather evidence to prosecute the man responsible for the theft. Even the Duras sisters might roll-over on Soran once he was exposed. That family isn't the most reliable, you know.
And in any event Soran could always break out of prosion years later and try the whole thing all over again.
So what? By that logic, no one should ever be arrested for attempted murder or arson because they could break out and try again later.
And Picard didn't even know his ship was destroyed.
Irrelevant. By acting to save his brother and nephew and capture Soran before he even left Amargosa, the
Enterprise wouldn't have been destroyed. It's ridiculous that by the final act, Picard had forgotten all about the information that had so devastated him in act two.