You don't need to be Marco Polo member to choose seats online. When you buy tickets online, you can usually able to select your seats already, maybe you have missed that part when you bought your tickets. But, if the tickets are promotional offer, there may be a chance that you cannot reserve seats before. I have bought tickets online directly from CX many times before and was able to do that almost every time. I would suggest not to take the seats close by WC as a lot of people will be around you (waiting for the toilet) all the time. For convenience, I usually take the one next to eisle, at least I don't need to say excuse me and ask for people to stand up to let me pass through. Also, the one at the first row is also good, especially for those who have kids as you have extra room for bags.
BTW, I'm an Asiamiles memeber and I'm not sure if it makes a difference for me to be able to choose seats in advance but I think that it's not. Since you have booked the tickets already, why don't you call the customer hotline and ask the operator to lock the seats for you?
花旗太太生活在花旗國,留意時差,你問我未必即時答到。