1 None that i know of ive had around 100 units at sea not including land units so i dont think theres a limit
2 it can do depends on your own system
3 reactor leakage or something yes see
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4 i dont think so once your dead your dead thats it.
5 why would you want to? A and D keys are handy in game, also the amount of time you spend at PD in a mp mission is so short the chances of getting them messages are slim even with the VLF wire. (from own experiance ive missed a fair few messages in my time)
6 not that i know of be amazed if there was, personaly the idea is not about speed and see who gets to point AbC first its more find and kill thats the idea of it.