No, there has to be inventory in the WU/RU buckets for upgrades. Just because there are seats open does not mean there is also upgrade inventory. If you were not upgraded, it means there was no inventory in the buckets and you will not be able to select seats for free just because they are open and you opted out. The part you are missing is that while you can select the seat, if there is no upgrade inventory, DL will ask you to pay for it.

For C+ on TATL flights, there is no separate fare class for C+ and there are no auto-upgrades to C+ (so opt-in/opt-out makes no difference on these). You will be able to select any open C+ seats at your appropriate window.