Yeah, I think you're right about the scrap being taken away. It was a decision I made for the sake of difficulty but in hindsight I think it just causes frustration. Being able to make adjustments as you continue to try to beat a level would be more fun than how I have it now. Doing something like you said, where the scrap purchases are not final until level completion is a fantastic way to keep the level of difficulty up, keep the "scrap economy" in tact, and also vastly improve the user experience.
I appreciate you taking the time to play and post this comment.
I hope you had fun :)