I haven't heard that they're using an alternate for Valjean in Toronto as they do often in London, but I don't know that midweek matinee would actually be the best one. Monday nights and at least one matinee are what alternates tend to cover, so I'd say Friday/Saturday nights probably safer as they are the big shows.
Toronto is probably harder than New York or London where there are websites devoted to cataloguing when actors are supposed to be on holiday (I used one a couple of years ago to align my trip to London to match with Alfie Boe's holiday week because ick!), especially because it's not an openended run either.
I don't know if Ramin is quite an active or interactive tweeter, but that's the sort of thing I would do - fire him off a quick message saying "Hi, we're really keen to come and see you as my bro is a big fan, do you have any time off arranged so we can be more sure of seeing you?" No guarantee you'll get an answer, but it's definitely worth a shot
But all that is for scheduled breaks, and sometimes, things do happen and that is unfortunately just a part of live theatre. Then again, understudies are often totally amazing (I still have a good laugh about people tutting in the audience at John Owen-Jones being off for a show, and then getting served up the phenomenally good Christopher Jacobsen instead <3 ) So by all means, go for Karimloo, but there's no reason at all to let it ruin the experience if you get there and he's down with flu.
Also, our own deHavilland is probably going to be a good source of info on the Toronto production, so once it's up and running, we might be able to find out more via her, since hopefully we can peer pressure her in to pretty much camping out at the theatre