Should all a plant's buds be picked at the same time? Can I harvest just the large center buds that have already matured? Will the smaller side buds continue to mature with 12/12 lights, or should I harvest the entire plant at once?

Harvest no bud before it's ripe. Yes, you can harvest the buds that are ripe, leaving the ripening buds more time and probably more light. Once the top buds are removed, the plant puts more energy into the remaining buds and they ripen sooner than they would if the top buds remained on the plant. Removing the buds before they are ripe is a waste of a whole season's effort for the impatience of a few days. To put matters in perspective, it will take the immature buds seven to ten days to ripen. However, there are circumstances that might cause you to harvest early.

The Weather: if it's going to be cold or inclement for some time, it may not be wise to leave the buds out to rot.

The Wrong People: if overheaders or on-ground hunters or uniforms are coming too close or have shown an interest in the area or the plants, its time to leave with the buds, ready or not. Immature buds are much more potent and worthwhile than no buds at all.

Indoors, you don't have those problems to worry about, just buds vs time. If there are only a few buds left, is it worth tying up space for them to mature? That's a question only you can answer.