The slight trace that results from sweat does not matter, and your prayer is valid. All you need to do is make sure that you clean yourself properly after relieving yourself, then do not worry about it anymore than you should after that, lest you fall prey to Satanic whispers.

According to the more correct of the two scholarly opinions, semen is pure. `Aishah said: I remember scratching semen from the garment of the Messenger of Allah (blessings and peace of Allah be upon) thoroughly, then he prayed in it.

There is nothing wrong with using a hairbrush made out of boar bristles, and it does not matter if it touches wet hair, but avoiding that is preferable.