Your intial hold on DL will be 14 days. They don't automatically extend it unless you call back. I've done this a few times with DL award reservations, but never more than one additional 14 day period so I have no idea if there's a maximum number of times that they will do this. Not sure about CO's policy.

Also check to see if there's a charge for changing dates after your award ticket is issued. I know DL doesn't (yet) but several other carriers have started to, just in case you don't get your first choice of flights initially.

And to answer your earlier questions, the 75000 mile award on AA is roundtrip for the 'plan ahead' (more restricted) award. Some carriers allow you to include interisland segments as part of your mainland-Hawaii award, some do not. Most of the time when they include interisland segments it has to be in direct connection with your mainland flights. I wanted to go from LAX-HNL (with stopover)-KOA-HNL(another stopover)-LAX earlier this year on NW. I had to redeem 35000 miles for my LAX-HNL flights plus an additional 5000 for my HNL-KOA flights because of the stopovers in HNL. I would call the individual airline and check. If it's not included, you can redeem 5000 AA or NW (maybe UA too) miles for one interisland roundtrip. Hope this helps.