I've got 2 questions:
Can you use your Retain tokens anytime or only during the Action phase?
The question arose when I drew the Finale card, I had a token that could give me back 1 Nemo (and a few extra VP), but could I use it while resolving the Adventure card?
You can use this effect at any time.
If you equip the Nautilus with magnetic mines and strenghened prow, do you gain +1 DRM when using the mines?
Thematically I'd say no... but rules don't mention it... maybe Nemo's found a way to also improve the mines
The effects are cumulative, so yes.