input for question 1:

'minimum permissible wall thickness' denote by tmin on section 5 symbol representation.
there is also wall thickness symbol with 't'.
if you refer to table 11 (im using API5L 44th edition) about wall thickness tolerance,
then you will find tolerance value both for seamless and welded pipe.

so...simple answer. tmin = t minimum with tolerance consideration
eg: seamless pipe with t<= .157" is t-.020"