You say your mother is executor - she is not executor until she is named as such. You also say there is no need for probate since it is so small. In order to cash checks made out to a deceased person or estate the executor needs to open an estate - If I am not mistaken for such a small estate in CA (I am not licensed in CA and so defer to the CA attorneys) there is small estate procedure which will allow for a simplified procedure to allow the qualified executor to deal with these assets. Typically an estate account has to be opened - this requires a document appointing the estate's representative (executor) and a TAX ID (EIN) number.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
You do not need Letters Testamentry or a probate if the estate is under $100,000.
A Probate Code 13101 affidavit will collect the assets.
Go here. http://www.leginfo.ca.gov/calaw.html
Click Probate Code.
Search 13100. Read 13100 and 13010 and 13105(2)(b).
Google for a form.
You cannot open a bank account for the deceased estate. You may try to collect from the banks the checks are drawn from directly. Otherwise submit the 13101 to the makers of the checks and ask for new checks, giving them back their first checks.