I'm flying to Europe next week on a ticket booked in January and need to come home a day early. Being less than 2 weeks out, the flight change is understandably way more than I would want (and no schedule changes that would let me change for free). I want to ensure I understand my options with DL if I cancel and book a separate ticket(s) with OAL miles:

1) Can I assume canceling the return after taking the first flight will almost certainly not give any credit as DL will re-price the ticket as a one-way?
2) Can I change the return to the cheapest one I can find to get some eCredit without any issues? Is it eCredit or some other DL currency I should expect?
3) Regardless of whether I cancel the entire ticket or a segment, I'm assuming the eCredit will expire 1 year after I purchase it. If that's the case, am I reading it correctly that the date is just to book a new flight by then and I can travel after the eCredit expiration?
4) If 3 is correct, am I correct that while I can't cancel the ticket and get anything back, I can make changes to that ticket AFTER eCredit expiration, but any changes resulting in a lower price will mean no eCredit for that difference?
5) If 4 is correct, how far can one continue making changes before the ticket simply expires?

Thanks!