Re: Regrading grabs in Aikido...
As far as I know O Sensei stressed ushiro attacks as a means of training awareness and power to move to the back of the body thereby extending martial capacity to a 360 degree circle. The technical applications are secondary. Of course waza are important but the chances of any attack or response being like a dojo scenario is miniscule. Body movement and stickiness to your partners center irregardless of their angle of approach is far more important.
However if you want to look at it a different way try this: throw jodan tsuki and have your partner sidestep, deflect and grap then continue to circle with the intent to grab your other wrist for either a takedown, or holding before using the knee to the spinal column. These are both scenarios which give some useful input.
Rear collar is eri dori and like kubijime can be considered to be "realistic", but don't get too hung up on realism in the dojo, its mostly overrated and misunderstood, better focus on seeing and "listening" to your partner.