The Clan Homeworlds have been said to be relatively resource poor, but at this point in time, those resources are only split 4 ways instead of 17. Each Clan gets a larger chunk. The Ghost Bears started working on the Leviathan in 3044, which means they thought they could do it with the resources that they had prior to the invasion. Sure, they needed some help doing it, but that was technical issues, rather than resource issues (in fact, in order to get the technical issues solved by the Snow Ravens, the Ghost Bears actually traded AWAY some of their resources).
The Nightlord may be more tenable as a lighter design, but its entirely possible that the factories and yards for the Nightlord were either blown up during the conflict, or packed up and moved by the Snow Ravens. We know the Leviathan yards survived.
So once the Clans have managed to build themselves up again they'll have more individual resources and the yards for the Leviathan. Why not build it?
Back on the topic of the Texas, the Texas was the ship I pitted against the Agamemnon to prove how borked BV is for Battleships. The Agamemnon fared well by exploiting weaknesses in the Texas's weapons arcs, but when the Agamemnon could no longer keep her range....the Texas obliterated her.
I'm not that surprised since a single hit from almost any of the Texas' non-missile weapons will actually strip the armor from half of the Agamemnon's locations, and leave the others with tissue paper in return. Of course, if the Agamemnon got in a few lucky hits with its NAC30 bays, the Texas could easily find itself bereft of weaponry. Not to mention I'm not sure the Killer Whales and AR10s of the Texas could get through the Agamemnon's point defense system.