Hi Brandon,
I'm a guitar player as well, and can offer the following, based on your questions:
1. Could I reliably use Mainstage live?
=> Mainstage is a pretty new software. Many people in this and other forums are using it without problems, but if you go through the archives you'll find people who don't. In my case, using Mainstage for guitar tones and effects, I get excellent results. You'll have to try and see what it does to you.
2. Does it have pretty authentic tones?
=> This will be a personal thing. In my humble opinion, they are very good. Other users have plug-ins such as Amplitube or Guitar Rig running behind Mainstage. I only use Mainstage's effects and models and get exactly what I want. Tweaking those models and effects may take a lot of time, but you'll end up with something you can use.
3. Is it conceivable I could turn effects on and off with just a button or click or two?
=> Yes. You may choose a floor midi board or some other controller, and you can assign a certain button on your controller to enable/disable (i.e., turn on and off) an effect. Or else, you can just do that on the laptop, but in a concert that may not be very practical.
4. I use just a few stomp-boxes and have a Presonus FireBox as an interface, and have the most highly upgraded Macbook.
=> Make sure you have a LOT of RAM memory, maybe your laptop can take 4GB?
In any event, you will have to do a lot of experiments until you get the tone/effects combinations you want. I've been tweaking my system for 2 months and, so far, I've got 3 ⚠ patches that I really like.
You may wish to look at a rack for the audio/midi gear and laptop. SKB and Gator offer 2 and 4-space racks with a top compartment for a laptop. That way, you have your rig wired all the time, all you need to do is plug the power cord on the wall and you guitar cable into it.
Good luck.
Mano