it seems when playing files from filesystem directly and ShowTime is enabled mocp is not showing tracks duration unless ^t ^t (twice indeed) is issued.

The '^t' key actually toggles (or "cycles", more accurately) through the values of the ShowTime option from your initial setting of "IfAvailable" to "no" then "yes", and this accounts for the behaviour you see.

If you want the durations at start up, then set ShowTime to "yes".

yes ^t cycles through possible options. ShowTime was set to IfAvailable ... by default as it was not set at all in the config file. But now it's done and to on and it seems to work fine.