To add on to when spectators ask to examine the card that is gimmicked, I always have a duplicate, normal card there for either a top or bottom change or any other switch of your pleasing. This is great and eliminates the suspicion because they really do handle the card themselves. It is a lot of crowd control though, that is the most important, but sometimes you come across a really skeptical and stubborn heckler kind of spectator. In those situations it is nice to have a way out such as the top change.
In terms of your fear of using gimmicks, just practice a lot. When you get the handling down so ou don't flash the gimmick, then you will feel more comfortable. This will help it seem like it is a normal card and there is nothing suspicious in your performance.
Basically, my tips for gimmicks is practice a lot, act natural and like yourself, and have a way out. If someone does catch it though, it doesn't matter. Just say thanks and walk away. There are plenty of others out there who won't know about it, and soon those people who saw it will forget the entire mess up and goon with their lives. Everybody messes up. I hope that helps you get rid of your fears.