1) If one’s hands are exposed to oso makom (ex: the ish’s hands after tashmish), and then they wash their hands with just water (ie-without soap), are their hands no longer shayich to a potential kesem issue? Meaning, can one assume that anything that may have gotten on their hands from oso makom was washed off, and therefore anything red can be assumed to have came from somewhere else? Or does one need to be chosheish that something may still be stuck on their hands from the exposure to oso makom, and therefore be subject to hilchos kesamim? What if they washed their hand’s with soap and water?
2) If the ish uses a (colored) towel to be mekanayach the eiver after tashmish and then afterwards (either right after or a long time after) uses that towel to dry their hands, does one need to be choshesh that something may have transferred from the eiver to the towel and then onto one’s hands? And therefore something red (even less than a gris) found on their hands may be a shayla? Or is this too unlikely and therefore one does not need to be choshesh that something like this happened?
3) If one first washes off the eiver after tashmish with water, and then is mekanayach with a (colored) towel, would a red spot on the towel that is less than a gris be a potential problem? Or do we say that since one washed with water first, it is not considered a situation of being mekanayach after tashmish, and therefore anything is fine?
Question 1 and 3 are very technical, and would depend on the individual circumstances of each instance individually. Therefore if something like this will happen it should be discussed verbally with a Rov.
Regarding question 2 This also seems quite unlikely.