Hi. I think you chose some good herbs for the active portion of your mouthwash.

I assume that you wish to avoid using alcohol. You could consider infusing your water with dried herbs in addition to the oils. These are all great antiseptic choices, clove oil can be very strong, but the oil blend will be perfect in the right ratio.

Other possible additions might be Calendula, Sage, Tea Tree, Eucalyptus and/or Cinnamon.
Comfrey seems out of place for me, but I've never become familiar with it whole, so I don't have a strong connection to it.
Your herb choices are great, and whichever herbs you choose, the important part is the balance of their concentrations, especially when you are dealing with essential oils.
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Is the baking soda there just to adjust the water pH? Too much can make it react with some ingredients, creating a fizz, usually In the case of acids, of which you have little to none (I don't think Mg2Cl lowers pH). I don't know how the magnesium chloride will mix.
To test the compatibility of your ingredients, It's always good to mix the ingredients with each other separately, or at least the ones you are unsure about. Mix them equally with your water and note any slight changes to the mixture.
You can test them at full strength (for solid ingredients like baking soda this is fully saturated water, but I guess, you could, just dump baking soda into an MgCl2 and H2O mixture). I recommend starting with a diluted mixture if you are unsure of reactivity.
With water as the main solvent of your mixture things should go well. Just be good to water, as it is good to us!
I hope your mouthwash is spectacular.

-Tyler