philippe willaume

To be fair paired technique with live blade is relatively safe. Yes accident can always happens, but parried practice
Even if the attacker had the choice of the technique, we know the side it is coming from, that there will be no feint, most of the time attacks are started out of distance and the attacker is striking out of his space. (But most of our attacks are, even with open hands).

Making sure that last to bit do not happen naturally and the will make thinks a bit more leery but still feasible.
I think that having our opponent staking outside of the is space is what we want to create but we should create it and when we get it for free we should know why we get it for free.

I have been doing medieval fencing for as long as I have been doing aikido, and so far I am not aware of a safe way to spare with a live blade (other that being armoured with defeat the purpose) regardless of the context.

You do not need a Shiken a shinai (and a fencing/kendo helmet will do the trick perfectly). And instead of being split in half, the worst most likely injury you risk is a broken collar bone, so your opponents can almost welly you to his heart content.
But that is sparing not form work. Sparing is there to so how you what you did not understand in form-work.

My understanding of form work and paired practice is really there to make understand how it works. So of course we are going to place ourselves in situation where it is going to work.
If we go back to the bokken, a fight is very rarely start in a bind like we do (ie with the weapons crossed), even with thrust/no cut to speak of weapon like small-sword or late spanish rapier. It is just a convenient way to start and make the output of our opponent consistent enough.
I feel like it is the same with any weapon we use in aikido.

phil