As regards seating... the fact that there are only three seats available for online seat selection does not necessarily mean that everybody else has chosen their seats, it probably just means that a lot of seats have been made unavailable for online seat reservation for one reason or another. You may well find that you can get two seats together by phoning KLM Reservations and talking to a human being - and the sooner you do that the better.

Are you sure YUL-AMS is always late? It's a while since I last took that flight but I don't remember there being any significant delays. If you are on a though booking then any delays are KLM's problem and not yours - if you miss your connection you will automatically be rebooked onto the next flight (and I believe there are plenty). 1:15 hrs is normally enough, but if you miss the connection just go to a transfer desk and KLM will sort you out.

As already mentioned you won't have to collect and re-check baggage, but you will have to go through passport control and re-clear security. If your inbound flight is late you may be able to use a special priority lane though passport control and security.

Just relax - you'll be fine. And welcome to Flyertalk!