Is it possible that the questioner had a miscarriage due to what she mentioned about her menstrual cycle stopping and brown blood coming down, and is she required to observe 'iddah because she wore her parents' underwear for fear of semen being on them, and does 'iddah become void merely by imagination?

All praise is due to Allah, who has accepted your repentance from these acts. Know that what you have mentioned are mere whispers (waswas) with no basis in reality. You are not required to observe an 'iddah (waiting period), nor were you pregnant and had a miscarriage. Rather, these are devilish fantasies from Satan to corrupt your life. Your marriage is valid. Discard these thoughts and illusions, and avoid succumbing to these whispers, for yielding to them leads to great evil. Be diligent in avoiding thoughts that arouse your desires.