Well, they aren't going to give you between 600 and 1200 euro for nothing... You can of course offer to be VDB, ask what the offer would be and then say no, I'd rather keep my place on this flight if you don't feel you are being compensated enough.

They won't know if they need to bump anyone until nearly everyone has checked in. You can offer to VDB at check in, but always check at the gate
too. Unfortunately doing it this way might make you seem too keen.

It's very unlikely you would get 'F' on an alternative flight and this was not what I was suggesting. My comment about getting an 'F' is that if 'C' is oversold some gate agents look kindly on passengers who offer to help them out when it comes time to giving out the red BP's

Unless 'C' is oversold and 'F' completely full they won't IDB a 'C' passenger, but if it is I wouldn't like to be the one who had offered then declined. As you say the compensation isn't that important to you, so I'd stick with 5D and get there as planned.