Open tickets tend to be a bit easier to change date/time/flights on than those that are ticketed. It shouldn't make any difference but sometimes it does.

I bounced around between LA and BA when I was in UIO this past spring trying to change a ticket that had been issued for specific flights. It was Saturday and nothing was open locally. LA SCL wouldn't change it because BA issued it. BA MIA wouldn't change it because they hadn't made the rez and it wasn't their flight (they wanted my to call BA CAI - right!!!). Lots of long distance calls for naught. Fortunately, the LA ATO handled the change the next day without any problem. If I had an open ticket all I would have needed to do was call LA and make the rez on the flights I wanted, giving them the ticket number I would use.

As for using the coupons out of order -- technically no but practically probably yes, especially if they are in separate coupon booklets or on different carriers.