For me, it isn't just that Delenn willingly gives power, it's also that Sheridan willingly shares it. He could, quite easily, take everything she has to offer and then use it for his own means and glorification - but he doesn't.

I agree. They both manage to acquire a lot of power... and then share that power with each other and the people around them. That should be hard to believe, but the script and the actors sell it: you believe these two characters are that capable and that good.