It would not be correct to say that mucous lining gets destroyed during jalaneti. As earlier explained in my reply to one of questions, though mucous is drained out during neti, mucous membrane again regenerates the mucous after some time and hence, the functions of thermal regulation and filtration from microbes do not get disturbed. As regards changing the nature of water to slimy so that mucous layer is not disturbed, this exercise will not serve any purpose of neti which is done using saline and lukewarm water.
Your concern for erosion of mucous /mucous membrane during neti is quite significant, but once a day thrice a week/ more for a limited period of time, with certain purpose may not cause any issue.
And the number of times of neti to be done depends on the severity of the problem. If you are feeling with acute cold, and sneezing, it would be better to do jala neti after you acute cold symptoms disappear.
Please feel free to write back for further clarifications, we would be happy to answer.
