You need to do both, they go together. Change to import(), and not use b-frames with ffmpegsource. Or, just copy the code from intermediatefile.avs into _source.avs; the effect is the same as import().
B-frames seeking incorrectly with ffmpegsource was a discovery. Using import() directly just make sense, since otherwise you're doing weird nesting like directshowsource(ffmpegsource(thefile)). Also, generally speaking directshowsource can be buggy and should be avoided if possible.
Like I said above, I'll patch in an import option to make this easier.
B-frames seeking incorrectly with ffmpegsource was a discovery. Using import() directly just make sense, since otherwise you're doing weird nesting like directshowsource(ffmpegsource(thefile)). Also, generally speaking directshowsource can be buggy and should be avoided if possible.
Like I said above, I'll patch in an import option to make this easier.