Hi there, In this context a synonym for "spoiled" could be "ruined" (or potentially ruined). What he means is that there is a problem or issue with one tiny detail of their plan - that they have not yet secured the services of the hat shop stunner. Because this has not been organised yet, their plan may not work out. Their plan is perfect, excepting for this one detail that could derail their plan if left unorganised. It isn't an out-of-date word in this context, but when searching for its definition it can be quite obscure. I hope this helps!

The one detail makes the entire enterprise impossible. So not managing to have the "hat shop stunner" help, means the the whole plan will no longer work. Spoil does not only affect a part, but the whole of the plan. It may be related to spoiled meat or food which makes it inedible.

It can be used for any situation. To spoil something means to do something so that the thing you planned before does not work at all. For example: 1. My kids wanted a party only for ourselves without adults. My wife and I had finished work a couple of hours earlier. We forgot there was a party and we came back home and we SPOILED everything (this is not a children´s party any more and they got bored so easily). 2. I have been playing a PC game and I had a very important task to do to complete the level. My mother has just called me for nothing...and she spoiled everything (I lost the points, I lost the connection and I could not finish the task). 3. Today I forgot to turn the facebook off and I suddenly decided to switch my PC to "away" and just to start studying. Some people started to type messages over and over and some jerks started to call without any prior notice... so they spoiled everything (it means I could not focus on my studies and at the end I did not learn much). Do you get it now?

This means reveal the truth as far as possible