When moving to an empty battlefield or when the last defender is removed, are there still windows for the opponent to interfere and prevent the conquer with actions/reactions?

Ruling: Attempting to conquer starts a showdown, and you can use actions or reactions during this showdown. The showdown phase is never skipped, even when moving to an empty battlefield.

Sequence:

• Attempting to conquer starts a showdown
• Players can play actions or reactions
• Focus passes around every time the chain empties
• Multiple actions can be played throughout the course of a showdown
• When a player plays an action or reaction that starts a chain, if the opponent has no response, it resolves and focus is passed

Nuances:

• You cannot use spells that aren't actions or reactions during a showdown
• Focus passes automatically after a chain empties, not through verbal agreement