Sumi-Gaeshi's key points
a) Can be done from a sleeve-lapel grip.
b) Better if done from a 2-on-1 grip, for control over one side of uke's body.
c) Fake a forward attack first and then sit down in a clockwise sacrificing motion.
d) Sumi-gaeshi can only be done in a kenka-yotsu situation.

Hikikomi-Gaeshi's key points
a) In an ai-yotsu situation, you can't do sumi-gaeshi (the body positioning is all wrong) but you can do hikikomi-gaeshi.
b) Reach over uke's right shoulder and grab the belt.
c) Tori's left arm scoops up and takes hold of uke's left upper arm.
d) Sit back and do a backwards roll over your right shoulder.
e) If the score is not ippon, make sure you pin uke.