The firmness of a pop bottle will not indicate the correct co2 level but rather indicate that it is time to open one of your bottles, check the co2 level and recap. If the cider is dry before you back sweeten reaching the correct co2 level will take longer than when bottling an active ferment. Depending on a lot of variables it could take a few hours to a few weeks for them to carb up properly.
Pasteurizing does not hinder the aging process in fact it helps speed it along. commercial wineries use heat to artificially age there product. Pasteurizing will not completely age the cider for you but it is a good start. Make sure you are familiar with the process and more importantly familiar with what causes bottles to break when pasteurizing. Here is a thread I started describing what I believe is a better way to bottle pasteurize. Another way to bottle pasteurize