It is trivially easy to capture screenshots from any screen, and the 'protection' that snapshot puts in place does absolutely nothing to stop this.
I would think twice before trusting that anything you post is not going to be widely available.
It's great for photos that are badly-lit, or inane, or make me look unattractive, or related to some sort of in-joke, or that only make sense in the moment. I wouldn't be mortified if any of them were released; they just don't make sense on my carefully-tended social media profiles.
The draw, at least for me, is not covertness or secrecy. It's a form of expression that doesn't demand the "does this really belong on my public profile?" test, which is really liberating.
Although it's trivial to take a screenshot, the sender receives an alert telling them which friend took it. Given snapchat is used for sharing pictures with your close friends, that puts having your Snapchat made public in the same probability range as having your mobile phone hacked and pictures posted online. Sure, it happens, but that's a risk people are obviously willing to take.
Snapchat provides a mechanism for trusting your friends now without needing to trust them for infinity.
(b) yep, as you said, even with the notification it is trivially easy for someone to take any picture that you send and post it to the forever webs.
So, I reiterate, do not send anything via snap chat that you do not wish to one day become public.