Excellent question and excellent points raised, I found also the following:
Some sages, like Ibn Ezra, suggest that God's acceptance of Abel's sacrifice was partly a divine mystery, intended to teach humility and trust. Genesis 4:7 ("If you do well, will you not be accepted?") implies that Cain could have been accepted with the right heart, but Abel's offering aligned more closely with God's will.
So regardless the kind of sacrifice if Cain approach God with the right heart and with just offering vegetables the account would have been very different. Both sacrifices would have been accepted