Perhaps what's niggling at me is the inexact overlap between Minekura's Buddhism and any other expression of it. Saiyuuki is only as Buddhist as it needs to be, for the plot or for atmosphere, but then hell, so is the novel. Minekura's is the Japanese home-made soup approach- throw elements in randomly as needed, for taste. Anything systematic is verboten but nice touches are to be applauded. (And then we westerners come along looking for systematic application and logic and are annoyed, while the Japanese wonder why gaijin never get the joke.)
Was Tenpou hoping to unlock and harness the power-potential locked inside Konzen? Was he hoping that by alerting Konzen of the spreading cancer in Heaven, the kami would upgrade to bosatsu, even?
That in fact is the only reason I can think of why Tenpou might have decided to alert Konzen (aside from info-dumping, I mean.) It still feels not-right that it would be by means of upgrading to bousatsu, though here I may be taking a too-technical western approach. Konzen need only manifest bousatsu power to be as-good-as-a-bousatsu, and as good as is all that's needed. I mean, I don't think bousatsu take exams to get there, except maybe in some Chinese novelist's settei. (M remarking that Chinese novelists regularly split their sides laughing over the idea of everybody behaving like good mandarins. The 'cue laughtrack' trope of Chinese literature. "And then he wrote a memorial to the Emperor, bwah-hah-hah!!")
Kanzeon looked to me to be merely opening Konzen's eyes to his own potential- stop being a dusty bureaucrat and start living already. But se may well have had an agenda of hir own. I do like the 'throw him downstairs or a few centuries till he's learned something' line.
But if we must think in a logical fashion then let's try thinking backwards. Perhaps Konzen is sent down to earth in order that he might turn into Sanzou. Who is what? A loose cannon, certainly. Good for shaking things up, not very good at rebuilding afterwards. Half darkness and half light, which might be a step down for Konzen if he's supposed to be Purifying Light. Not so much ineffectual as choosing not to be effective. (Sanzou can be very effective when he puts his mind to it, as he did in the Hakkai and Gojou section of the burial arc; but he doesn't often put his mind to it.)
Sanzou was sent to the west at the start at least partly to get his values straight and learn to trust a few people (if Minekura hasn't forgotten that bit) which suggests that he too still needs to learn something. However, canonically from the novel, this is the incarnation that has reached the point of being able to go back to Heaven. Konzen the bored bureaucrat is the start point; Sanzou the testy neurotic is near the end point. Sounds to me like Kanzeon just wants Konzen to turn into someone who can kick butt.
Unless se wants someone who, when offered Heaven, will refuse it. Which strikes me as likely- 'But thou, meek lover of the good!/ Find me, and turn thy back on heaven'- but still doesn't shed light on what Tenpou may have been after.