When you, issue the shutdown immediate and still if you see oracle not available, then it would be the time to just abort the oracle instace through shutdown abort...

SVRMGR> shutdown immediate
ORA-01034: ORACLE not available
SVRMGR> shutdown abort
SVRMGR> startup mount
SVRMGR> recover database
SVRMGR> alter database open;


This would be the way to go...

Sam