That is correct. Also take note that some networks or programs may place your cookie for less than the 30 days. This is another factor you have to consider when choosing the network you want to work with.
This is techically possible but I don't know if anyone is doing it. It is possible to read if a cookie already exists and then don't overwrite that one, so it would be the first cookie set by the first affiliate that counts, because it won't get overwriten.
It also makes sense (in most cases) that the last affiliate's cookie would override the original one because it's the last affiliate who is actually doing the successful pre-sell - and therefore he/she should be the one getting credit for the sale.
how about those cleaner or optimization software on our desktop? e.g. CCleaner, these stuff clears everything, including browser cookies. is that going to be a problem??
if our targets are tech savvy people, surely most of em have something like this on their PCs. i know i do...
Of course it's a problem. If someone uses it when he has your cookie and then buys the product, you won't get any commision. Well, I seldom clear any cookies, they keep me logged in to places