Is Quebec's proposal for assisted suicide law legal? Likely not.

I am writing legally. This note does not consider the "morality" of the issue. 

First, the Criminal Code (Federal law) provides:

241. Every one who
(a)counsels a person to commit suicide, or

(b) aids or abets a person to commit suicide,

whether suicide ensues or not, is guilty of an indictable offence and liable to imprisonment for a term not exceeding fourteen years.

Quebec law that directly conflicts with Federal law is legally not effective. Federal law has paramountcy over provincial law. 

Accordingly if Quebec says you can aid a suicide and the Criminal Code (which is Federal) says you can't then the criminal/federal law takes precedence. 

What about the Charter of Rights and Freedoms? Could that uphold the Quebec law? 

Probably not. Rodriguez v. British Columbia (Attorney General) [1993] 3 S.C.R. 519 upheld the ban on assisted suicide in Canada despite strong constitutional arguments. 

Now as a matter of federalism the provincial government in Quebec prosecutes criminal cases.

Quebec might decline to prosecute doctors who follow legislation allowing for assisted suicide; but that's a grace and favour sort of thing and might well be challenged. Besides, a provincial policy not to prosecute could change any time and there is no limitation period for serious crimes in Canada. 
So, legally, Quebec's proposal seems a non-starter without Federal agreement.