It's been a while I haven't posted here, and I'm a bit surprised how come people haven't answered your questions...
I will try and help you:
a) "Don't sit too near at the edge of the chair, you may fall down"
It's much better if you say: "Don't sit too near the edge of the chair...." without the preposition "at". It's both wrong and longer:)
b) How to say, if you want to pass by a place..but many people are standing there..Can we say,"Can you give me a way?" or "Excuse me, can you give me a way"?
The shortest way to say it is: "Excuse me, please?" , and trying to pass. People understand your intention, and you don't need any extra words.
And if you need help, please post again, I will try to be more present here.