Is my Preliminary Notice valid if it went out unsigned under the Proof of Affidavit section?

The "Proof of Affidavit of Service" does not need to be sent as part of a California Preliminary Notice. Rather, this is meant to be kept for your own records if a mechanics lien eventually needs to be filed. The affidavit is essentially a sworn statement declaring that the notice was sent out to the listed parties, by the appropriate means, and on the date listed. If you do eventually have to file a claim, this affidavit should be included with your claim. (be sure to sign it then!)

So the short answer is, yes. The preliminary notice will still be valid if the Proof of Service Affidavit wasn't signed.

Here's some resources you may find helpful:

• The Ultimate Guide to California's 20-Day Preliminary Notice 
• How to File a California Mechanics Lien | A Step-by-Step Guide