General questions on channeling magic through daggers in 4e

When a sorcerer uses an a dagger to channel a power with the implement key word, what bonus does the dagger give them? The phb 2 says you don’t add the proficiency, but it also says you are proficient with the dagger.

Also, how do you calculate critical damage when channeling through a dagger? Do you include the daggers crit damage (if it’s magic or basic)?. How do you find it’s crit damage?

And finally, can you have a magic item or weapon of a higher level than you, and use it to channel spells????