Should I be trying to please all my platesters??

This is a tough cookie to crack! It takes a good amount of fortitude to take criticism of your game, regardless of whether they are right or wrong in their assertions.

To have an effective playtest session, you should know what kinds of players you have playing your game, so that you can validate their feedback against their expectations of the game. As an example: a player that prefers mastery over strategy is going to have a hard time with chance-based games, and will most likely feel 'cheated' by dice rolls or pure luck. Does this make their opinion invalid? No!, but you can temper their feedback - which will still be useful - with the knowledge that your mechanics aren't favourable to that player's style of play.

Some of my reactions to your questions:

This sounds like a question for the players who don't question the balance of the game. These players may find it easier to comprehend the system, but that doesn't mean there aren't flaws in your information layout that are aggravating your fourth player.

Perhaps ask your two positive players if the information presented to them in the game coincides with their understanding of the game's strategy and systems, or if their understanding comes from their experiences with similar games.

No! Everyone who plays your game will have valid feedback, even if they're not your "target market". It's a matter of asking that person the right questions so you can glean the good constructive feedback beyond their initial motivations for playing the game. Maybe your gut instinct about your information flow could be correct. Or maybe this fourth player is trying to tell you there's a pacing issue. Or perhaps something else you haven't considered yet!

The point is the players don't know your game as well as you do, and therefore they won't have the same vocabulary and knowledge of your systems as you do. If someone complains about balance, that is just a symptom - the reason for your playtest is to find out the cause, and feedback from all of the players can give you the answer your looking for. Maybe it really is a balance issue, but if they can't put their finger exactly on what is making your game unbalanced, then the problem could be somewhere else in the game that they don't realise.

There's a danger here to create a playtest group that can easily overlook flaws and dominant strategies because they "understand" what you're trying to do. Getting constructive criticism can be hard - see the note up the top about fortitude! - but it is all necessary to make a better game. (Adding more playtesters may not be a bad thing to do as well!)

Having said this, it's important to understand what YOU want out of your game. As I mentioned previously, you will understand your game's vocabulary and systems better than anyone else, and you will know what your overall design goals are. Your sovereignty over your game should be what drives you, and let the feedback of your playtests pull you up when something gets in the way of achieving your design goals.

(As an aside, I did a presentation on this very subject last year. The slides might be useful if you are interested.)

Good luck!