Mechanic's lien questions described in "situation section"

California mechanics liens effectively expire 90 days after the lien was filed. The deadline to enforce a CA lien is within 90 days of the lien filing. So, if more than 90 days pass and no enforcement suit is filed, then the lien will no longer be valid and enforceable (though, it won't automatically disappear from the property record). More on liens and lien enforcement deadliens here: Do Mechanics Liens Affect the Property Title Forever? Of course, note that California does allow for this 90-day period to be extended if the lien claimant and the owner can agree on it. Further discussion here: California Mechanics Lien Extensions.

As for your second question: First, retainage is lienable because it represents amounts owed for work already performed. Plus, generally, claimants are able to file more than one lien against the same project. So, filing a separate lien claim for strictly retainage is one way to proceed if you haven't been paid for the work you've performed. But, keep CA's mechanics lien deadlines in mind. For one, the deadline for filing a CA lien is 90 days after project completion. And, once the lien is filed, a claimant will have 90 days to enforce their lien before the lien expires (unless the lien is extended).

Keep in mind that all the amounts owed and unpaid for the project can be lumped together into the same lien, as well. So, if you wanted to file a new lien including retainage and past invoices and to release the old lien that's about to expire, that could effectively restart the 90-day enforcement clock. But, before releasing a filed lien, it'd be wise to make sure that the second lien could be secured before the lien filing deadline.

Additional resources

These resources should be useful to you:

• California Mechanics Lien Rules & FAQs
• What Is “Enforcing” a Mechanics Lien? 
• The 4 Steps to Take After Filing a Mechanics Lien