Weak alcohol treatment is mild. Formaldehyde in the fixing medium could occur as polymers. The effects of formaldehyde fixation on protein may be on the uncharged groups of the protein; I don’t know if there is any major effect on charged or protonated (-NH3+) groups. Fixed proteins appear to retain their secondary structure. Tissues fixed in formaldehyde (for cytochemistry) appear to be normal when looking under a microscope. It may also depend on the concentration of formaldehyde, and the duration of fixation.