In the “ good old” pre invasion/pre taint days of the clans, I am curious how trial of possessions worked. For example:
Clan Fish develops the EER ( Extended Extended Range) PPC and Clan Slug wants it. How did deals typically go?
In this case, Clan Slug would declare a Trial of Possession for it. They could trial for copies of the weapon (easiest), for the design specs (harder), or outright for the scientists who made it, the factory, all examples, and exclusive control of it (hardest).
If Slug and Fish are allies would they attempt to trade?
Allies, or even just two Clans who each have something the other wants could trade. After the general terms are agreed to by the merchants, there's still a series of Trials. The two sides agree on how many lots to fight over (5 was a popular number), then conduct separate Trials for each individual lot. The vast majority of the time these Trials ended up going 3-2, both sides come out happy with something they wanted and one side gets a little bit more.
Would a trial of possession happened right away if they were neutral or enemies?
Neutral sides could still made trades as I described above, outright declaring a Trial for another Clan's tech or other resources tends to be viewed as a hostile act. The Clans didn't really care about ticking off an enemy, so those were almost always straight Trials of Possession with nothing offered up in trade.
How does Clan Slug find about the new weapon? Did pre – 3050 clans have Watches that could let their Clans know that new tech was abound?
While the Clans had some form of Watch units (varied by Clan), encountering new weapons of equipment on the battlefield was generally how things were discovered. From there information was a resource that could be traded/trialed for if a Clan knew who to ask.
Could the merchant or scientist cast bring a petition to the warriors to trial for said new weapon?
If there's a new weapon, the warriors are going to be the ones to discover that, short of an information trade. Scientists aren't going to be allowing such info to slip into the hands of another Clan, that would in effect be treason.