Best wishes!
Making an economically collapsing nation a tributary won't be that profitable, unless you are just planning to bleed it dry, but that would inevitably lead to that country growing resentful and getting closer to your enemies, should the enemies be willing. If you know how to fix their economy, though, it could be profitable indeed, not to mention you would gain an ally. One nation fixing another nation's economy isn't that common, though, but this is a work of fantasy. A reincarnator could also use modern knowledge to do it. Maybe make that potato country Duran a giant of French fries, potato chips, and potato vodka.
That's kind of how I interpreted it as well. And at that point, they're not really formulating concrete strategies, but rather just talking lightly about it, so some simplicity is expected. I mean, other than this being a manga that doesn't go into international politics too deeply.To my understanding the idea of getting these countries under their control is for the bigger goal of isolating Laverl.
The direct benefit they get from saving Duran isn't big, but by causing Duran to be on their side instead of Laverl, they can chip down more of Laverl's power without having direct confrontation.
Is "everyone love liz" is because her pure personality or because of some charm or magic or because the saint ability like in the bertia? Its kinda amazing thats she is like a pampered ojousama pure flower (u know what i mean) despite she is being a commoner