I don't see any contradiction or problem there, and I doubt that the authors do either.
1. A "post-scarcity society" is a society where resources are no longer scarce relative to human needs. It's so cheap to provide everyone with the best standard of living that there's no longer any reason to compel people to work for it. It can be given away essentially for free, like wifi at a cafe.
That doesn't mean that every resource in that society is infinite. It doesn't mean that the society can afford for everyone to build giant mechs and smash them against each other all day. It doesn't mean that blueprints are perfectly free or that there are no onerous restrictions. And TIME is just as scarce in Lancer as it is today: you get one second for every second that passes. (Sometimes less.)
I also have no idea why resource abundance would eliminate trade. People have been trading stuff for a long time and will probably still want to trade stuff when they have more stuff.
2. Sure, Union is not that far from an empire. Under the Second Committee, it was an "empire in all but name." It's certainly not pacifist or particularly shy about imposing its values. And what's your point?
There are some authors who use their fictional works to validate their own ridiculous prejudices and dogmas. If they have an opinion, they write a novel where the people with their opinion are the best and good and smart, and everyone else is cruel and stupid. If you're coming from those authors, I am not surprised that you think the lack of an overriding, absolute economic and social dogma in Lancer is an error on the part of the authors.
But it is not an error. Union is not the best country with the best economic system and the best opinions that are the author's opinions. It is a realistically flawed future state.