Have only connected int'l - int'l at YYZ once - and it was a real pain. This was UA to BR late in the evening BTW.

Had to go through immigration (i.e. enter Canada), landside and again through security. Is this the standard approach in Canada?

I think both options are fine. LX J is perfectly fine if you get one of those individual J seats. Or at least I've picked those seats on the A340-300 in the past.

Of course AC J is ok too.