A: While cylinders and rings work, because of friction and how fast you have to spin, it might be impractical and take up resources that can be used to power or fuel other parts of the ship. Also if you travelling very fast, unless the ring speeds up to compensate, the gravity may be disrupted. The thrust to get gravity is not an option for us mainly because we do not have engines strong enough to give us gravity. However they would be practical as no extra construction is needed for gravity and in space you will be travelling a lot. Rings are our best option longer duration missions, but i the future if we have superior engines we could use the thrust generated gravity.
B: I don't think it matters if space combat will be practical, it might happen or it might not depending on the technology we have. But i think battles will be like how the expanse portrays it. Torpedo barrages, railguns and such and then point defence. However pretty soon we are going to probably have lasers instead of the expanse's auto-guns.
C: I agree, it completely on how our civilisation developed and grows into space, including on how space-travel is accessed by the general population, piracy would would become a problem if space-travel becomes a commodity like cars. Earth is not going to come together anytime soon, so each nation is probably going to have independent forces, bases and colonies in the Sol system. But if earth unites there will be less conflict, but not none at all. If space-travel is restricted like piloting a plane and such, the future will include a lot less space-based conflicts.
D: I'm talking about near future space-designs, not what ships would actually look like in a interstellar federation and such. If we were to find alien life or get diseases and stuff, i don't really know. If we send a sort of generation ship to alpha centuri for example (With rings or a big cylinder of course), It would probably be passed by a superior ship sent decades or so later. About other civilisations, i also do not know, but we might not find another one due to some sort of great barrier such as nuclear war that kills off civilisation before they get to a interstellar stage. We might of passed this, or we might have not, which is scary. If we meet a friendly empire, that will be beneficial to both parties if we co-operate, but if we meet a xenophobic empire and they want to purge us wel,l oh dear.