"A lot of mugwort tea" concerns me. You should be careful about dosing. Just because it is a bath doesn't mean it cannot enter through your skin. It also contains tannins, which means it has the ability to dye your skin. It is a sedative (mild) but that doesn't mean the strength of it won't increase with an increased dose. It is a diaphoretic, which means it can cause you to sweat more. It is a diuretic too, which means in combination it could cause dehydration if you're not careful.
When using any herb, just because it is non toxic does not mean it shouldn't be used with caution.
Mugwort isn't even desired as an aromatherapy scent, so its purpose in your bath, from what I gather, is its magick properties. Since that is the case, a little can go a long way. Don't go overboard.
Phew! Now that I've gotten that out of my system. =)
I think your bath will be a great experience. I love meditating in water, it is very soothing. Allow the energy of the water to cleanse your energy. Don't let it go to waste. It is there and it will be empowered with the herb and crystals. Let the energy of it seep into your body slowly, finally filling it, and as it fills, see in your mind this beautiful positive energy pushing out the negative energy into the water surrounding you. See this water brimming with power overtake the negative energy, cleansing it, causing it to turn positive itself. When you're finished with your bath, let's the energy return to the water.
I personally wouldn't allow it to just go down the drain to mix with dirty sewage. After you're done, remove the stones and then you could send the energy of the water into the earth where it may do some good. When the water is just plain tea water, pull the plug on it.