How mechanically inclined are you?
If you can do the work yourself, I would first take out the plugs and see what they look like. Are any fouled? Is there coolant on any of them? Are the black and sooty or white and ashy?
This will be able to tell you a bit about what's going on in your combustion chamber.
If the plugs look good, then check the COPS. Pull the plugs out and see if you get spark. If you find a plug that doesn't spark when it should (or at all), then replace the COP.
After you've done this, you can be pretty sure that your spark system is working (yay!). However, that means the problem is something else (boo). And it also means it could be much more expensive to fix (double boo).
Since the shop was talking about smelling coolant, and you are getting random misfires every few days, I would start thinking about the head gasket. If that is ruptured (say, around cylinder 2 and 3), then it would slowly leak coolant into the chamber. This coolant buildup would make it burn coolant when starting up, as well as occasionally foul up a plug and cause it to misfire.
Unfortunately, a head gasket is not a cheap thing to replace at a shop. You have to disassemble a good portion of the top of the engine, and most shops will charge you a lot for that. Fortunately, the gasket itself is only like $30, so if you have the skills and the time you can do it yourself.
As for the other potential causes, an exhaust leak will cause a rich condition, which I suppose could potentially cause a plug to misfire if it got really, really rich. But I think you'd notice other symptoms too (gassy smell, water dripping out of the tail pipes, etc). I would be surprised if the leak is causing all of these problems.
To summarize, I would start with the plugs, then move to COPS, then decide if you want to do the head gasket.