My reference for sleeping bag selection is to use one that is rated at least 10F lower than the temperature in which you expect to be sleeping. So...if you think it will be 30F, use a sleeping bag (or bag + tent/bivvy) that is rated at 20F.
During the Summer in the Sierras (not So. California) I use a Marmot 40F bag, plus bivvy sack or tent as a backup so I can sleep comfortably at 30F. In the Fall, a 20F bag. In the Winter a -20F bag, etc.
The peeing problem can be helped (at least for guys) by sleeping with an empty bottle (wide mouth!). Then you can pee into it without getting out of your sleeping bag. However, make sure the pee bottle can be easily identified by touch, so that you don't make a mistake in the dark...
There is really no excuse being too cold when having ample time to plan ahead for whatever might come your way.
But if you get into a forced bivvy, where it is not possible to safely descend, then, as I heard from an Austrian mountain guide: "Suck it up, princess..."
