on further research, this problem has been around for well over a year and has been reported multiple times - I don't understand why it hasn't been fixed in the release code by now, as it appears to have an easy fix, as in an earlier closed (but not acted on) bug report at
http://wordpress.org/support/topic/plugin-w3-total-cache-call-to-undefined-function-after-update?replies=3#post-2308798
(the second error in my post above appears to be unrelated)