Since I now spawn, hatch, and feed train bluegills in one of my ponds I think I can answer your questions. But for those of you that have more technical info please chime in (i.e. Eric West, Bill Cody, Bob Lusk, CJS2005 etc.)
how long before a newly hatched bluegill will start to feed on a pellet diet?
Not sure about the time duration but I've got them feeding on trout starter diet at about a 1/4 of an inch in my bluegill production pond. I believe that is couple weeks after hatch but could be wrong.
When bluegill start feeding on a pellet diet, a couple times a day do they still feed on invertebrates and zooplankton or do they just fill their stomachs on pellet feed and stop eating when full?
Yes as far as i could tell, anything that fits in their mouths is fair game especially when I'm not feeding them. Depending on their size they can feed 24/7 (the really small fish) to only once or twice a day for the largest bulls.
If the majority of bluegill mainly feed on the pellet diet, in contrast this will leave more zooplankton and more macroinvertebrates meaning more organisms to feed on algae which would mean a clearer pond correct?
"It all depends." Depends on density of fish as to how much grazing they will do. I wouldn't depend on it especially in a recreational pond with predators.