Re: ECJ and ECHR
ECJ: highest court one can appeal to regarding EU law. Agreeing to this is required for EU membership.
ECHR: Highest court one can appeal to regarding Human Rights law. Agreeing to this is required if you wish to be a member of the Council of Europe (nothing to do with the EU - it is a separate organisation). However, in order to be an EU member, you must also be a member of the Council of Europe.
So, if a country wishes to no longer agree that the ECHR is the highest court regarding Human Rights, then that country also loses membership of the Council of Europe; in in doing so, it automatically cannot be a member of the EU.
Russia seems to be doing its best to lose membership, having passed a law that makes Russian law supreme to the ECHR; and by not accrediting a delegation to the Parliamentary Assembly of the Council of Europe.