The other thing you are doing wrong is triggering any ability once you start counting attack and defense STR to resolve the challenge.
Once defenders are declared and all players pass on taking actions, the challenge starts to resolve. In order to resolve the challenge, you count STR, determine the winner, settle claim, award unopposed (if applicable), and award Renown (if applicable). Then, you resolve passives related to winning/losing the challenge, settling claim, etc. Then, players can trigger Responses to any of that. Once ALL of that is done, you can start playing player actions (like Ambush) again.
So in your original scenario, once you started counting STR, you were out of chances to do anything with Ambush, period.