First, Thanks for asking those questions, even if it's silly it rare that women get some interest in the male sexuality.
1) more volume of semen does make the orgasm more pleasurable. When I orgasm I feel a tickling in the gland that travel to the base, if it's not strong enough and just stay in the gland I can slow down and carry on that's "edging". If it does reach the base, the base muscle start to pulse and nothing can stop the orgasm. The difference between a big orgasm and a normal one is the number of pulse the base muscle do. It could do a lot of pulse with little semen but it tend to be longer if there's more semen to expels.
2) During the orgasm, it's the pulsing of the base muscle matched with the tickling in the gland. the gland become over sensible at that point and you just want to stop to touch it. Peeing don't have the tickling prior in the gland, it's more of a constant pressure coming from the base that release a pressure on the bladder. it doesn't use the same canal in the penis, but it does procure a nice feeling. it doesn't have the pulsing that make the difference.
Maybe the fact that pee and semen don't use the same canal and that the canal of semen is closer to the center of the penis where you got the most sensible part do play a role in the intensity of the orgasm. And the fact that when the penis is erect it's really difficult to pee, so I could say that when I do cum the pressure is more intense then when I pee.
Hope that can help you understand.