>IBM wrote DOS
No.
Gates bought DOS (originally called QDOS) from some dude for $30k and licensed it to IBM. QDOS was itself sort of a cheap ass rip-off of CP/M.
Uhm… 20/20.
Interpreter, yes. Not compiler.
Really any 1.2MB or 1.44MB drive had r/w heads for both sides. Early variants came single sides (just one r/w head), I think 360KB for IBM PC format, IIRC. I've never personally seen a drive that addresses both sides individually (A:, B:) tho.