I am not going to answer the questions directly, as partly due to personal circumstance, my answers would not send the message I would like to send in my response. However, I would recommend you game plan for as many games as possible. If that is too much, focus on just one team, or just two, instead of all 3. Its better to really dig deep in one team, or in one system, than to scratch the surface of many.
There are a few reasons to game plan: 1) the immediate impact on your % chance of winning the upcoming game, 2) the information you can gain about your team that will help you set them up better later in the season, and 3) the information you gain about your system and the game engine that will help you plan and set up all future teams better.
The debate you hear all the time is pretty much over the first reason - the impact on your chance of winning the upcoming game. Its true, in general, that the changes a coach makes on average for a game don't have that big of an effect on the outcome (although, certainly not negligible). However, the changes a coach like OR would make to a team's setup for most other coaches, given the opportunity to coach 1 game for them as well as possible, would generally have a really significant effect.
So, when you are game planning, and observing the results after wards, keep in mind the two other reasons I mentioned above! You really want to get your team in tip top shape for the post season, as well as building your game planning ability. Also, by playing with game planning settings a lot, you can expose details about the sim engine - make a small change, observe a small result, that is how we can comprehend things. If you don't focus on game planning, then you are basically comparing the outcome of whole seasons, and the changes in your team and the teams you play from season to season are huge. So its practically impossible to separate out the causes and effects at that level. You want to make changes at the lowest level you can, to give you the best chance of figuring things out.