If you clip just the finished buds, sometimes a new bud will form in its place!

On leaving most of the upper plant intact:

1) Rule of Thumb for Crop Plants: Leave leaves whenever you can. These are your plant's solar panels. When you remove some, you cut down on the amount of energy your plant will get.

2) It matters less than you might think whether or not the lower buds are getting direct light. Flowers (buds) themselves don't do any photosynthesis, and the small leaves around them do very little because of their size. One plant is a single organism - The energy produced via light captured by your large upper leaves will feed other parts of the plant!

3) If you really want more light on your lower plant, you can buy some cheap clamp lamps, and put in some warm or daylight CFLs, or even (oh god, here comes the barrage of naysaying!) incandescents (which produce much more of the red & far-red light plants need during flower) to supplement your overhead HPS.