Gaining access to an Oracle RAC instance that fails to start due to problems introduced by initialization parameters is trivial *, assuming access to at least one other working instance in the cluster.
The solution requires creating a parameters file from memory in one of the running instances, starting the failed instance using this file, and overwriting the system parameters file that contains the unwanted parameter values.
The following statements on the corresponding RAC instances will do:
create pfile = ‘/tmp/pfilerac.ora’ from memory;
startup pfile = ‘/tmp/pfilerac.ora’;
create spfile = ‘+DATA/RAC/spfilerac.ora’ from pfile = ‘/tmp/pfilerac.ora’;
* Trivial as in there is really no complexity in this, and also trivial as in it took me, a non-DBA, a couple of hours to figure it out.