Having kits doesn't affect the cancer rates at all - like for humans, being a virgin or not doesn't affect your health except in terms of venereal diseases which animals can get too. Like for humans, having youngs can affect the rabbit considering she can die while delivering, but it has no long term effects on her global health. Does get cancer because when they are available all year around, without cycles, which means they have constantely an insanely high level of hormones coursing their body. They just are not engineered to last long, which makes sense when you take into account the number of things which are out to get them - wild rabbits never reach 5 years anyway so it's more important for the species that they are able to reproduce fast and often rather than having individuals who can potentially live for long. Rabbits aren't particularly sensitive to anesthesia or antibiotics. Antibiotics can slow the guts a bit, but it seldom really affect their health, and if gas is used by a competent vet death during anesthesia is a rarity (it's around 1%, considering that most of the deaths are actually caused by a heart problem that wasn't detected before the operation). If you compare it to the 80% of cancer risk, I never hesitated a second to get my rabbits spayed.
To give water, you just give it with a syringe, like meds. But don't do it if your rabbit is not deshydrated. You have to adjust metacam / simethicone / critical care by watching your rabbit. Nobody can tell you if she needs them without seeing her - if she's not in pain, you can lower the metacam. If she's not gassy she doesn't need simethicone. If she's eating fine on her own and that her weight is maintaining itself, she doesn't need critical care. If you are not sure, you have to discuss it with a vet.
