Hi there,
Just adding my two cents.
I am currently raising 5 Cobb500's for eating. The plan is to eat them somewhere between 6 and 8 weeks. They are 2 weeks old today and they are HUGE!!!
On the salt water issue, this process is known as brining, and is a way of making the meat moist and imparting some extra flavour. You only want to brine a roasting chook, if you are planning on jointing the chook and cooking it by another method (such as frying) brining will put too much moisture in the meat, and you will end up losing it all again in the cooking and stew the meat! Usually you brine a chook 24hrs before you want to cook it. If you do it before freezing, you may find that it will take on a more watery taste, although i haven't tried brining before freezing. I do know, however, that this is the way we get some of our frozen supermarket chooks, often labelled as "extra juicy" or something like that. I'd say extra watery! So I'd be more inclined to brine after defrosting the chook. My very vague recipe is to get a bucket and half fill it with cold water. Dissolve one cup of salt, and half a cup of something sweet (sugar, molasses, honey is nice) some peppercorns, and any spices you want to use (I like cloves and star anise). Add the chook, and top up the bucket to full. Fridge overnight, then make sure you rinse the chook really well, pat it dry and oil before cooking. Works really well on the Christmas turkey!
On the slaughtering issue, I haven't done it before. My plan is to do all the chooks on the same day (so they won't miss each other) and to not do it in front of each other. This is more for my benefit than theirs. I am going for the sharp cleaver method. Anything you read about spinning the chooks around first, or other crazy things, don't believe it! I have been asking around, and using crazy methods to "put the bird to sleep" is just plain distressing to the chook.