Who files the tax returns for the estate in the event of the executors unexpected death?

It does not sound as if any income tax returns or estate tax returns would be due yet, given the timeline you lay out in your post. An estate tax return, if one is due at all (it may not be necessary) is not due until 9 months after the date of death. An income tax return would not be due until roughly 3 1/2 months after the final day of the estate's income tax year, and an estate can use a fiscal year that ends on a date other than December 31, which means that if death occurred in December 2020 the estate's first income tax year could end as late as November 30, 2021. All that said, a successor Executor needs to get appointed for the estate. The successor Executor will then need to file any needed tax returns for the estate and for the decedent (whose 2020 income tax return will be due by April 15, 2021 if no due date extension is requested, but if you can't get an Executor appointed more quickly than that the return will have to wait until the Executor has been appointed.   No one has the authority to file a return for the decedent or for her estate until a successor Executor is appointed. The fiduciary for the deceased Executor does not have the authority to act with regard to any other estates.

Apply to the court to appoint a successor executor:  either the person named as successor executor in the Will or, if no one is named, someone willing to serve.  That successor executor will be responsible for filing the tax return and have authority over the bank account.