Let me first start by saying that, in my opinion. Sarah Silverman is very hit or miss. I find her jokes either hilarious, or I just don't get it at all, with very few falling somewhere in between. That being said, everyone knew what they were getting into when they wanted her to host. You don't hire George Carlin for a 5 year old's birthday party and then complain when he uses "dirty words" in front of the children. If you're likely to be offended by a performer of this type, don't put yourself in a position to let them offend you.
I feel that the reason Sarah didn't do well, was not because of the fact that she was hitting people while they were down, it was because it's all been done before, and quite often at that. We've heard all the jokes, and yes, while they were funny to begin with, over time the endless variations begin to get a bit tiring.
Moving right along to Britney. I can't blame her for everything, but she has to shoulder a lot of the responsibility for where she is today, and to feel sorry for her is to only enable the behavior that got her where she is.
She, along with others like Michael Jackson, Lindsay Lohan, etc. became famous, before they were mature enough to handle it. Now, some of the blame should fall to the parents, because in a situation like that the parent should be able to help the child deal with the overwhelming fame, and if it's clearly not working then a responsible parent would prevent their child from being in the spotlight, effectively letting the fame dwindle away, possibly to be picked back up at a later date when the child is more mature and ready to handle it.
However, there comes a time when you have to grow up, and you can no longer blame your parents, therefore you MUST accept the consequences, no matter what they may be, for your actions. Britney made the choice to get married and become a mother, knowing that it would put her career on hold, if not destroy it outright. Now that she has children, she needs to be a mother first and foremost, and her celebrity can come when she's got the time to deal with it, and if that time never comes, so be it.
Feeling sorry for someone who can't get control of their own life does no good at all. She's got the means to get help, and there are people out there more than willing to give it freely, all she has to do is accept it. There comes a point when you almost have to give up, she's been getting worse and worse, and soon it will be too late to help her out.
Now, if she truly is trying to turn her life around, and her "comeback" was a part of that, it was handled in the worst way possible. Assuming that she was to do some fantastic show involving the "oldschool" Britney dancing, and illusions designed by Criss Angel, that would have been wonderful. If, as the story may be, she was forced to change her entire performance almost last minute, she didn't handle it. She could have refused to perform at all, and simply done it someplace else. On the other hand, she could have made the best of it, and come out and performed like it would be the last time she would, and blow everyone away. Instead, we some half-assed, "I can't be bothered to make this decent," performance, which really falls on no one but herself.
At this point, she should just retire, raise her kids, and hope for their sakes that they don't end up in a similar situation later in life.