1) Cancellation. You will get a DL E-Cert for the amount less TWD 3500. You can use this on any flight (or flights) purchased though DL.

2) Change. You will owe TWD 2000 + fare difference. It does not matter if you are in the same fare class or not. [Perhaps you were confused by the "-FARES IN DIFFERENT RULES ..." clause near the end. This is wrt a ticket that has multiple segments with different change fees]

3) Seats: Have DL give you the CI pnr and do it yourself regardless of whether or not DL would do it for free. If you use DL to pick the seat you are simply adding a middleman on the phone call, and much more likely to end up with a mistake.

Question: Is there a reason you are booking this on dl.com? I though using CI prices out better than the DL codeshares?