You're not over-thinking it. I too was worried about the repercussions of accepting an offer only to retract it later on. The way I see it though is, if a school is going to give you a limited amount of time to accept, which was the case for me with the University of Calgary, then what do they expect? I accepted the offer because they gave me 1 week to decide. Yesterday I sent an email pretty saying that another opportunity more suited to my needs has come up and thank-you. At least I let them know right away and it's still early.
Bottom line is this whole process is a bit of a game and you try to be considerate and ethical, but are sometimes left with few options. And at the end of the day, you have to think of yourself and what's best for you.
Also, some Canadian Universities are part of that April 15th deadline. I have attached the link for you to check out:
The first link is about what the Council of Graduate School is about:
http://www.cgsnet.org/Default.aspx?tabid=201
And the second link lists all the Canadian Universities that are members:
http://www.cgsnet.org/Default.aspx?tabid=102&ctl=InstSearchResult&mid=437&TYPE=REGION&VALUE=CANADA
And just to rant a little, why is it that this whole process isn't more regulated? I think each university should have an agreed upon deadline and on this day, decisions need to be made and sent out asap, and from that day each applicant has 2 weeks to make a decision. For instance, every Canadian University has to send out decisions by March 15th...then you have 2 weeks to see what your options are and make a decision...and after that, the waitlisted people can find out where they stand. We have deadlines for other things like taxes...this would make so much more sense. As it is, I won't find out about some of the other schools I've applied to until mid-April, possibly longer whcih leaves me with no choice but to accept an offer I may later retract.
Good luck