Just returned and am updating with info from my experience.
China Airlines
- Seat reservations were available through the Delta online tool.
- "Preferred" seats (aisle in the front half of Y) were not available online (as Sky Team Elite +). They showed as occupied.
- Selected seats "stuck" for the weeks up until online check-in. At online check-in, one of two seats had inexplicably moved. I moved it back without issue.
- Upon checking a bag at TPE, counter agent offered improved seats (aisle bulkhead).