I disagree, I cant understand how a Military Press could be seated, kind of loses its meaning, I dont think you could ever be at attention seated...but maybe thats just semantics.
While you are correct in stating that there is no leg drive in a MP, an OHP maybe just a generalized term of any standing press. An explosive over head press would be a Push Press where you are using leg drive. I know of many people who switch to these types of presses as it can be easier on the shoulders.
But then again, I consider myself more of a PLer who does adopt some BB movements. While it is not included in the PL meets some feds have a strength meet where a MP is included, which I plan to enter in eventually.
If bodybuilding is your goal, and you have the aforementioned joint issues, I dont think the MP or OHP has to be a mainstay...I would think you can use other alternatives for the same effect. But take my opinion with a grain of salt because Im not a bodybuilder, but I like to hangout in the thread lol!
anyways good luck