Why don't you have internal password? Can you login as a member of the dba_group, do sqlplus /nolog, conn / as sysdba?

If you could log on, you could try dropping the tablespace including contents, assuming this was just an index tablespace. Then recreate it.

The only "switch and bait" file manipulation you can perform is for identical files - internally indentical files, that is, as far as Oracle is concerned, like redo log and control files. Datafiles, unless multiplexed (obviously, yours was not), no can do.