Verifying old blocks
when are old blocks verified by each full node? 


never unless rescan=1 command-line option passed to


  From what I've seen, the possible answers include: when it's downloaded and when you do a -rescan.


yes, on downloading also of course

one of my .dat files is broken (some garbage in contents) but bitcoin-qt works well because it uses UTXO, not blocks themselves


  if I change the contents of an old block (and thus break the hash chain)


this depends what do you mean.