To be honest the importance of masters series tournaments is overstated. I doubt many multiple masters series champions actually remember how many masters series tournaments they won in total, or their individual records at each of the individual events. I know that Sampras and numerous other players from the 90s have no idea how many masters titles they have to their name.
Really its a septagon, but the olympics is different to a slam or a regular tour event, in that a gold medal is a gold medal regardless of whether it has been achieved in singles or doubles, so Federer is an olympic champion. Similarly the Davis Cup is different in that it is a team event that combines singles and doubles (often the crucial and decisive rubber).
Exactly, see for example Phelps or Bolt, they are individual racers but also do team efforts and those medals are just as worthy as the ones they got on their own
Also, from the current big names, the only one with two Olympic medals is Federer.
Mirnyi is an Olympic champion, same as the Bryan Brothers, Rafa Nadal, Fernando González...