Update
After a couple of sedations with two rounds of bloodwork, urine samples, etc, the vet has concluded that Lunar's kidneys are failing. They don't know where he would fall on the continuum, as there is no established scale/baseline for this in guinea pigs. About the best they can do is extrapolate based on dog/cat situations. It does equate to a shortened life span, but no idea on the length. He could live another month, another year...no clue. They also don't know what causes the kidneys to fail; most cases are confirmed post-mortem, and there are no obvious common risk or pre-disposition factors. There is nothing they can do. The organs are so tiny, and guinea pigs' health is quite fragile, so any medical action (including biopsy) carries an extreme risk which the vet and I agree is out of the question. All I can do is take good care of him.
It frustrates me that so much is known about guinea pigs' health tolerances in relation to their being lab animals (and do I ever hate to think about one of these little guys being experimented on), but little in relation to their just being guinea pigs. There's hardly any baselines to measure against. And he's just a little 2-lb pig...he's never hurt anyone or done anything wrong...that frustrates me too, to know he suffers without having done anything at all to deserve it.
As I told a friend last night, I am of course not looking forward to the inevitable, but am trying to enjoy the now. Easier said than done, especially when today I keep crying when I think about it. But, I will keep trying.