The catch rate is significantly lower if you don't hit the mutli-colored circle at all (outside the white circle), from personal and others' experience.
This question is a bit subjective because it comes down to your own confidence in your PokeBall throwing abilities.
If you are consistent, I would recommend going for the 'Great!' throws, which are harder to hit than the biggest circle, but still boost your catch rate significantly more than the "Nice!" throws.
If you are very confident in your abilities, go for the "Excellent!" throws, but these are the hardest to hit (as you mentioned). Sometimes I like to practice on "trash" Pokemon (low level Pidgeys, etc.), where the stakes are lower, so I can get better at my "Excellent!" throws.
If you are not at all confident in your abilities, stick to the "Nice!" throws, as they are the easiest to hit and reward more xp and catch rate than hitting outside the white circle.
Another note: This is a bit obvious, but the closer the Pokemon, the easier it is to hit the harder throws. My current strategy is I try for "Excellent!" throws when the Pokemon is close up, because with a little practice you can hit these fairly regularly. As the Pokemon backs up (if it escapes), I go for "Great!" and "Nice!" throws, depending on the distance. I also change my strategy based on how many PokeBalls I have. If I only have a few left and I'm trying to catch a more valuable Pokemon, I go for the riskier throws, because I don't want the chance of the Pokemon breaking out and me running out of balls, but this is, again, subjective. If I only have a few left and I'm trying to catch a less valuable Pokemon, I go for the safer throws, because I don't want to miss and waste PokeBalls when the Pokemon escaping isn't as much of an issue.
I also don't try curveballs anymore, as to the extent of my knowledge, the xp bonus does not stack on the xp throw bonuses, but this will likely be fixed in a future update.
EDIT: As noted by @gitsitgo, curveball xp bonuses now stack on throw xp bonuses, as seen here.
The gym leader of the gym (or as you state, the lead defender) is decided only by CP. The strongest CP Pokemon is the gym leader. The order of Pokemon you battle in an opposing gym corresponds to ascending CP, starting with the lowest CP Pokemon and ending with the highest CP Pokemon, which is the gym leader.
Here's a relevant article that explains how to become a gym leader.
Best Answer
Maximum CP is determined by both IV and trainer level.
Since trainer level is the same for all of a given trainer's Pokemon, a trainer can choose to look at only IVs instead when determining which of their pokemon have the highest potential.