I've been off a while so will respond to (I hope) all of your points at once.
It's a given that randori is not availible in many places because the bulk of Aikido dojo don't do it in any formal sense. I'm not saying that's good or bad but it is the reality. Unless you're doing tomiki or some similar variant, aikido simnplydoes not focus on sparring. Which isn't to say it doesn't happen in an "informal" way amongst like minded students. To some people this is Aikido's downfall, to others it's it's selling poing <shrug>. I'd guess the majority of the people on this list do not do much in the way of sparring (in the sense we're talking about) during formal class time - that's why I take it as a given.
Of course there's no rule about the sempai training with the kohai. We operate a satellite dojo, with gradings being performed by our shihan in at our hombu. Some people manage to get to hombu/seminars often, others are more confined to the satellite. For these latter people the hombu instructors and students are unfamiliar.
I understand where you're coming from, as someone who dabbles in BJJ I have come to understand the value of sparring and it's an aspect of my cross training I really enjoy. Whether we should spar in Aikido and if so how that should be set up is another thread and one which I suspect has been done before. My point is simply that given that many many dojos don't spar, grading is all you have left to observe technique in an environment that is more stressful than day to day training.