I am not sure I can completely answer all your questions, but I will try on the ones I know. If you use store bought water I think distilled water would be even better than other water anyways. It has all the chlorine and other minerals and salts out of it so you have a clean water source with no contamination. Your pH I would check at least once a day. There are all sorts of things you can use to bring it up or down. Easiest thing to bring it down (acidic) is just something like vinegar. Baking soda is an easy way to bring the pH up (alkaline). You can also buy pH up/down solutions at some plant places and also places that have fish stuff should have for fish tanks. I am not sure about the seedlings and stuff but I think at that stage when the roots arent present as much the growing medium soaks up the nutriets for the time being (not sure though completely, guess).