... except unlike with counterfeiting, when all is said and done (all debts paid and all bank balances withdrawn), the total amount of US dollars in this closed system you describe will still be $100. You are ignoring the balance sheet of the second person (the one the bank lent $80 to) and only looking at that of the bank and the initial depositor.
No. Suppose the $80 are immediately paid back. The bank just moves the $80 from the loanee's checking account into the bank's own checking account. Total amount of money is now $180. $80 have been created, and is now owned by the bank.
Edit: Wow really, someone downvoted this?