1) You do not need to add any more yeast. There will be plenty still in suspension. You only need to think about adding yeast if you are close to the abv limit of the yeast, or have been conditioning the beer for about a year.
2) Extra yeast will make no difference to that amount of priming sugar you should use.
3) 4 ozs of corn sugar seems low. I assume you used a calculator to get that. The temperature you enter in the calculator should be the maximum temperature the beer has been at after fermentation has slowed down, not your lagering temperature.