Solved

Is there a gap between last race and designs being released for next season?

Ok, so a few questions.

1. Does only the CD affect the next season's designs, or does the TD do that as well?
2. Is there a gap between the last race being completed and the designs being released for next season? (i.e. I can buy a new CD and it would have an effect)

From the Help section in-game under 2.1 about Next season design:

"Generated per-race by the current active Chief Designer"

There is no gap. Hiring a new CD after the season ends would not affect the new season design.

Thanks! Probably should've researched further before wasting your time... lol