Unfortunately, whatever should not happen is happened. One shouldn't drop a datafile no matter whats the situation unless they want to blow the tablespace off.
I guess,You don't have any solution other than restoring the database from previous cold backup and loose the data from the cold backup time.
Dropping the datafile leaves a pointer in the database tablespace. Whenever you try to bring the database up it looks for that datafile.
Not sure. Give a try. Try copying the original physical file you found to the location where it was and try bringing up the database. If there was no activity on the datafile since the file is in readonly mode, it may work out. Just a trial.