Do not denigrate the benefit of looking like you know what you are doing.
First off, show business is a stressful business. The actors and musicians, not to mention the directors and producers, have a lot on their shoulders. So don't add to their tension by making them doubt the sound will work. Keep your doubts to yourself. You know the solutions you came up with are compromises. But the actor doesn't need to be thinking about that when they are on stage.
Of course looking like you know what you are doing is nice from a business standpoint as well.
Looking like you know what you are doing is nice for the audience, too. It makes them feel as if they are in good hands, and will be presented with a worthwhile night's entertainment. That is, assuming you deliver! Otherwise you risk having them think to themselves, "All of this equipment, and the sound still sucks."
Don't underestimate the placebo effect. Sometimes an audience member may wonder if they are hearing the third guitar. And they'll glance at the microphone set up in front of it and decide that they must be. What they don't know is that the guitar sounded horrible or the mic blew a gasket or for whatever other reason it isn't connected to the sound system. But you leave it up there because it looks cool, and it convinces the guitar player and his friend in the audience that you are serving him.
Don't get me wrong. I'm not advocating dishonesty. But in most cases, expressing more confidence than you have is a good thing. We are engineers, after all. Engineers know how many things can go wrong. But from a statistical standpoint, the sound will probably work. So you burden yourself with the fears of all those things you know about and no-one else does. And let the performers get on with the task of worrying about the things they know about.
Escape the temptation of perfect optimization.
Very few problems ever have a perfectly optimal solution. You can go crazy -- and you can waste a whole lot of time -- weighing too-similar alternatives. Just pick one and commit to it. Any loss you suffer because the alternative actually was slightly better will be, in most cases, completely offset by the time you saved by going ahead anyhow.
No plan survives first contact with the enemy. But the right plan will keep you alive long enough to come up with a better one.
Recognize that what you intended to do is not what you will finish up with. When you finally see the real set, when you finally hear the actual band, you may realize a lot of your work was unnecessary. So be it. It is foolish to try to hold on to work that isn't needed anymore, and even more foolish to search for justifications of why you still need it. It is also, more subtly, foolish to blame yourself for wasting time. If you front-loaded the work as much as possible, you wasted that time when there was more of it available to waste. And, beside -- you learn as much if not more from what didn't work, as you learn from what did.
The trick is to stay flexible, to not get too emotionally attached to what you thought you were going to do. And to design in such a way so you will have that flexibility when you need it.
This happens through doing many things, large and small. Document, and self-document; when you have to make a quick improvisation, it helps immensely if you can figure out what the existing system does. If you need to grab a cable quick to use somewhere else, you really, really want to know you are grabbing the right one; the one you no longer need.
Piling everything up in one huge mass with no labels and no organization and no documentation sets you up for failure in every case but the rare one when the event actually does unfold exactly the way you expected it to.
Have spares. When you are under the stage, run a second cable just in case. When you are running a power line, break it out with a strip just in case you need to plug something else in there. When you run out cable, dress the slack at the business end, just in case you have to move the microphone. Cover your bases, and anticipate having to make changes.
You can't use the air above you, the runway behind you, or the fuel that's back in the truck.
Bring all the gear to the gig. It's always the one piece of gear you were sure you didn't need, that you do.
Leave a spare snake cable. Test the mics before the curtain opens. Load in the night prior if they give you a night prior; don't count on getting it all done at the last minute.
You get the picture.