I've had to re-cook a batch or two. No big deal, and for some technques, char cloth that's not completely charred actually works better. If you're holding it with your thumb against the flint, you can blow it into flame by itself without other tinder.
As for the "pinhole", I'd advise making it at least finishing-nail size, and no larger than an eighth of an inch or so. The only caution I'm aware of is that you get a fair amount of pressure buildup in these things. With a tightly-fitting tin like a candy tin, the bottom (which is sitting on the coals) often heats and expands faster than the top, sealing it even tighter until it cools. I've gotten pretty impressive gas jets coming out of some of these (especially if it catches fire), and a couple of times I've enlarged the hole because I was afraid the thing might "pop" while it was cooking. That could get interesting- I envision small squares of flaming cotton scattered randomly over the surrounding area...
I was told, way back when, to blow the flame out if the gas jet caught fire. I don't know if it really makes any difference or not- hard to see how it could, unless the flame got inside the tin somehow- but I still do it.