Mouthwash on Tisha Be’Av
I know that many poskim hold that on tisha be’av one should not rinse ones mouth with water out of fear of perhaps swallowing the water? What about mouthwash in which no one swallows? Why should this be worse than washing dirty hands? Dirt is dirt whether its on your hands or mouth.
As you rightly note, there is a distinction between water, which a person might swallow, and mouthwash, which people are careful not to swallow anyway.
Therefore, under conditions of (severe) discomfort, it is permitted to use mouthwash. This should not be done without discomfort, because we prefer not to place anything in the mouth on Tisha Be’Av.
The following is a mistake:
You may wash your mouth with water only if not doing so would cause great distress. Mouthwash on Tisha B'Av is a no-no. (On other fast days, rinsing with water is permissible if not doing so would cause even a little discomfort). It is important to make sure that you do not swallow any water during rinsing and it is therefore recommended that you rinse while bending over.
In conclusion: Mouthwash that is not suitable for drinking is preferable. The great heat tomorrow is certainly reason enough to be lenient (preferably with mouthwash, and, in any event, with the proper precautions) on washing one's mouth to maintain a sense of hydration.