I don't have all the answers to your questions, but although pears are usually considered safe, I've heard some moms say it worsened reflux for their LO's. I'm assuming it depends on the variety as well as the ripeness of the pears.
I *think* oatmeal is less constipating than rice.
Another question about the vomiting - usually when solid foods cause increased reflux, it would be more through acidity and therefore pain, than with actual vomiting. Is there any consistency difference with the pears and carrots vs peaches and other foods he's ok with? He could be very sensitive to textures?