Hey Rogersa! I agree - you need to take a deep breath, buy from one of these reputable places we've all mentioned here (Tiffany's, GOG, Blue Nile ...) You have to put the word "warranty" OUT of your diamond-shopping vocabulary and replace with "home insurance rider" and just leave it at that.
Okay so I'm a Tiffany's girl (I'm in Toronto too) and I have to say the service is exceptional. You can walk into Bloor St. any time to have it cleaned, checked, and even polished back to brand new (band and all) for free within minutes (cleaning/check) to hours (polish to new). When your boyfriend says "they'll do it for free..." it's SORT of true... obviously part of the considerable mark-up there is covering that service and convenience (it's very relaxing to be able to take what is probably the single most expensive item you own in and "drop it off" knowing you'll get the same one back that day without doubt, and you sound like someone who would worry about that!! I would too honestly) you get from them... but to many of us that is worth it in the long run. Also, they have a lovely upgrade plan!

Not that everyone is into that but it's nice to have that in your back pocket!
Incidentally I've had very small diamonds (.5 I think?) replaced in my wedding band twice and it was about $60. Not a big deal.
There are tons of reputable jewellers in Toronto that can service your ring! But I'm going to bring up the obvious now that I know where you live... what about Birks? You can pay probably $20 every six months to have Birks check any diamond ring for you if you're nervous and trust that they'll take care of it! As I'm sure you know they're highly reputable and have a terrific service dept. at Bloor Street.
Also, have you considered buying from Birks? Not as high a mark up as Tiffany's but great quality and service... also, my sister had her independent-jeweller-made diamond e-ring re-set by them (something Tiffany's doesn't do) in their classic four prong setting for about $800 or so, so you COULD definitely buy a diamond from GOG and have it placed in a Birks setting if you like Birks, and I'm pretty sure that would put you in their service plan (but please check! I know for sure you can choose a setting there and "bring your own diamond" if you wish).
Buy for the diamond and the setting... then have it insured.