Agreed! As I have already mentioned, those holding to the RPW too frequently focus upon the "elements" of worship and not upon the "motive" of worship. Consequently, you hear arguments like you reported where those holding to the RPW violate sound biblical hermeneutics and fall victim to eisogesis. Grain offerings and other similar offerings which were not animal sacrifices were surely acceptable throughout the OT. Abel's offering was acceptable NOT because it was according to some undisclosed
command given by God. But rather it was acceptable because it was offered in faith and thus Abel brought the "
firstlings of his flock and of the fat thereof". In the Hebrew, it actually says that Abel brought the
fattest firstlings, i.e., the best of the best of his flock. Contrariwise, it is not said that Cain brought the best of his crop nor of his first-fruits. The reason was due to the antithetical dispositions of these two individual's minds and hearts.
Secondly, there is not even a hint that Abel brought a "sacrifice for sin" whatsoever. To insist that this was the real purpose in Abel killing an animal to present as an offering is to transpose that which is found in the expiatory sacrifices of the Mosiac law. These sacrifices brought by Cain and Abel were offerings of gratitude, albeit Cain's was only so perceptually and not actually.
Lastly, the RPW does NOT depend upon any necessity that the reason that Cain and/or Able brought an offering to God rests upon divine command. Both of these differing offerings flowed from each man's respective state of mind and heart. As I also mentioned above, there is more than ample biblical warrant to support the RPW. One must be faithful to the inspired text and not impose an unwarranted truth upon any passage which is unrelated to it. We must always seek to first understand the original INTENT of a passage before trying to seek to understand its MEANING. To err in the first most always guarantees we will err in the second.

So again,
faith is no less a necessary element of the RPW as is the
expression of the RPW.