Possible that a similar method is used, of distributing parts of the Imperator's armor, only said armor is equivalent to a mastework set of plate rather than being dracheneisen.

As for what makes a person worthy to become an Eisenfurst and thus be granted land, that could very well be up to the Imperator.

are you asking how  Nick Trague and Falk Fischler came to power, or how they became Eisenfurst? Those aren't the same question. 

Page 38 has some details about Eisenfurst: the original Eisenfurst had that title as they originally controlled mines that produced dracheneisen. those mines are now exhausted, so the title has lost it's original meaning and now simply means the ruler of a konigsreich. 

Trague and Fishler do get brief bios on their rises to power on p39 . As for why Trague and Fishler are Eisenfursten, that's a little difficult. It's entirely possible that the last Imperator granted them the title of Eisenfurst and created them new Konigsreichs, carved out from pre existing ones. That would create a very interesting tension between the "new" Eisenfurst , and the "old" Eisenfurst, as they would consider Trague and Fischler illigitimate, as they do not own Drachenesien mines. 

I was venture the area Fischler holds was just a factor of needing more local control. (Either to secure a border or difficulty for the previous management to reach and oversee it.). Prague may have been given Freiburg to keep the others in a power balance or, given his military history, he may have just claimed it at a time when nobody was strong enough to say no. I could very well see him deciding enough was enough and he was putting a stop to the local part of the conflict in the war of the cross by assuming control and refusing to let any side hold sway.

Of course, they could have access to dracheneisen mines and they're just being very quiet about it . They're just giving all their out put to the kreuzritter. They don't want the information public because it would start another war