i. STPM is valid forever. well, atleast till u die. however, most fulltime undergraduate courses in universities have maximum age limit, which is about 27-28 years old. if u're above that, u will enroll under mature-student programme.

ii. i'm not sure. however i believe it's the university that will blacklist u, not UPU. meaning if u withdraw from university A for no valid reason, u can still apply for university B. do check.

iii. refer to my previous answer. it all depends on the reason of withdrawal.

iv. yes. obviously.