1) The GRE score is not mandatory.
2) Contacting your potential supervisor is definitely helpful.
You must apply for the thesis option to get a supervisor and
finaltial aid may be considered for these students first.
3) If you are applying for master's program, the department committee
will decide who they will accept but your supervisor will have a say.
I was in the same building as the CS dept., so my information may not
be 100% accurate. You can check www.queensu.ca for the department
information and the profs email info and research areas are all there.
Just keep trying and you will finally get it.