[EvilEvilKitten]

Unlike Brandye, and with apologies to her, I have no medical training, yet I can orgasm from stimulation of the clitoris, the G-Spot (of its existence I have no doubt), and the posterior fornix. Not only do I know it can be done because of my own personal experience orgasming, but also from having taught men who then go out and teach other women and so on. Within the swing community there is not only no doubt but men with the right skills are in high demand.

Yes, if the man knows what he is doing, and the lady is both relaxed and aroused, G-Spot caressing using the penis is possible. G-Spot orgasms vary in intensity just as all other orgasms do.
Yes, you feel like you want to urinate. When you get more experienced you will learn to tell the difference between feeling like you want to ejaculate and the feeling you have to urinate - there is a subtle difference.