Sorry... I should have qualified it by saying solo acts.. I totally agree with your statements. If I've got 3 vocalists, I would prefer the same mics on all.
No worries. I would certainly entertain a soloist's desire to use their own mic, but sometimes it doesn't go well and it's not always a simple matter to quash feedback. So while I think their is evidence the OP was dealing with a hack audio tech, it's also possible that the system was well prepped and swapping in a different mic created an unfortunate situation that was difficult to overcome.
My guess is the vocal channel had too much compression dialed in. This makes it hard for the vocalist to hear their voice and also makes the mic very sensitive to feedback. Unfortunately the last time I mixed as a guess engineer, I got burned by this when we had to transition one of our vocalist into monitor wedges in the middle of a gig because her IEMs were taking huge RF hits.
The monitor mix sounded great when I soloed it in my headphones, but she kept complaining that she could not hear herself, and I could not turn her up any more. I thought perhaps she had just become spoiled from using IEMS, but I spent a good bit of time trying to dial out the feedback so I could give her more. Eventually I figured out that she had two compressors engaged on her channel and both were being too hit hard because I had adjusted the gain on all of the vocal channels. As soon as I adjusted the compressors and renormalized the gain on her channel, all of the feedback and balance problems went away.
