and save back the changed content.xml into the .ods archive, your Sheet1 should be unprotectd. What did happen to you when you tried?table:protected="true" table:protection-key="K7O251GdsyeKg54HeoSQ5ak3goc="
As Villeroy already stated changing "true" behind protected= to "false" would also do. However there was a typo in his post:
instead of"protected=false"
Was this your problem?protected="false"