I see this thread was started in March, but others have responded in the last few days, so
I will chime in and hope OP sees all of the replies....
Was there today. The "skip the line" ticket skips the line to get in, but, if you've also purchased
a ticket for Pala d'Oro and the Museum, it doesn't skip the line for that. There was no line for
the Pala, but a fairly significant one for the museum, mainly because the stairway is narrow and
they mush everyone together who wants to go to the museum. But if you want to see the ceiling
mosaics much closer up, get the museum ticket, as it gets you to the second floor. The museum
has some restored mosaics which you more or less can get right up close to.
As for the lighting, I was there around 11:15 and the lights were already on. Was still there when
they turned off the lights around 12:45, and the difference is huge. You may still be able to see
the mosaics near the entrance well, as they are near windows that let in ambiant light, but the
major ones more to the interior of the basilica are very dim after the lights go off.
Yes it's crowded, but it wasn't that crowded today. And you're taking a lot of pictures of things high
up, so that mitigates things a bit.