We don't really need to provide eye witnesses to the rogering of Heaven, though. Witnesses to the aftermath should be plenty.
Zathir as a friendly and personable deity, Yrael as an Archon with a high position within the Imperium, maybe that Shaitan dude whose name I cannot recall who is now one of our high officials, Elaheh who once lived in Heaven's Shore, and Mereth as a highly placed Devil, the first who swore themselves into our service, in fact.
That covers a good range of non-Human witnesses, IMO. As for Humans, maybe Zherys and Lya would like to give their accounts of Hellven and the mechanics of the present day cosmology?