Yes 

The camera itself is not waterproof, so it can’t be used underwater. The protective case, lens hood & UV filter will protect the camera from the elements like snow and rain. Prolong use of the camera in extreme weather is not recommended. Like most of the electronic products, the battery life will decline in extreme cold condition. Contact us for the advance skill of using our camera in snowy weather. 

I have the backpack myself because it is so "unobtrusive" as a photo backpack. Meanwhile, I stow two Sony bodies with lenses (a macro and a 70 - 300 mm from Tamron) and a Minolta 28 - 80 mm in it, so your Canon should pose no problem. 

I have a Canon and with 300 that's just the way it is. 

No sorry it won't. This adapter is to use Canon EF and EF-S lenses on a Micro 4/3 camera body.