The rulebook specifically states that you can choose not execute a War Advantage that you won.
Rulebook, page 22, Resolving BattlesIn addition Seppuku states the player has a choice several times
Only the player who wins a War Advantage is allowed to execute it. However, the player may choose not to execute a War Advantage they won, in case they only wanted to win it in order to deprive another player of it.
Rulebook, page 22, Seppuku
The player who wins Seppuku may choose to immediately kill all their figures in the Province. If they do so, ... If a player decides to perform Seppuku, ...
And yes, you can play Betray even if you can not replace any figures.
Rulebook, page 18, Betray"Up to" means you can choose less or even 0.
When playing Betray, the player chooses up to 2 figures on the Map, ...