The niṣṭhā ta is changed to na after śyai but not when the participle denotes 'cold'.,
Thus शीनं घृतम्, शीनो मेदः, शीना वसा; but शीतं वर्तते, शीतो वायुः, शीतमुदकम् ॥ The य् of श्या is vocalised to ई by 6.1.24.
The prohibition applies when the noun is an adjective and means 'cold'; and not when it means a disease. Therefore we have प्रतिशीनः with न change.
The word स्पर्श is a guna word formed by घञ्, and denotes the particular guna or sensation to be sensed through the organ of touch. In this sense it is derived from the root स्पृश संस्पर्शने ॥ It also denotes a disease, derived from स्पर्श उपतापे ॥ There is nothing to show, what स्पर्श is meant in the sutra. Explanation is the only refuge here.,