Tap water has carbonate dissolved, but all of the other trace components of the water act as a buffer, so it stays around pH 7, while distilled water is substantially more acidic because it lacks the non-volatile buffers. It's possible that it'd still work at a higher pH... Couldn't hurt to give it a try (if it doesn't work, you've still got a dose of mescaline cooked down to a single shot and ready for consumption).

I wouldn't try adding any citric acid until it's been verified that the method works as-is. Depending on how much you add, some could end up in the final product increasing the dosage (and making the mescaline quite tart).