Clearance: Direct to airport with /U

The other day I was flying to 3N6 in NJ from JST in PA in a plane with two
VORs and a VFR-only GPS. About halfway through I got an ammended clearance
giving me a shortcut. The last point in the clearance was ARD then Direct
3N6.

After accepting the clearance, and turning toward the next waypoint, I
browsed further down on the chart, and realized that they omitted the
feeder fix (RBV) in the ammended clearance. There is only one approach into
Old Bridge, and the IAF is an intersection of RBV and COL, not ARD.

It was a clear VFR day, so I didn't make a big deal of it, but I mentioned
it to the final controller, and asked him to verify the vector that I
picked off ARD, at which point he cleared me to RBV anyway. But did the
controller who ammended the clearance without RBV make a mistake? Was it my
mistake for accepting it? In IFR conditions, with a comm failure, would I
have flown to ARD then RBV anyway? Should I have asked the controller for
an ammendment as soon as I noticed the error? Or am I mistaken in recalling
my instructor tell me that I can't fly direct to an intersection without
some sort of RNAV unless I am first tracking one of the airways that make
up the intersection.

Thanks!