Yes, it is difficult to get the Vakya Bhashya of Adi Shankaracharya on Kena Upanishad, since most of the translations only covers the Pada Bhashya.
I've found one pdf book that contains only Vakya Bhashya Sanskrit text of Adi Shankaracharya with English translation on Kena Upanishad from Chinmaya International Foundation
You can download the pdf which ...
Agni is central in all vedic rituals. All the offerings in the Vedic sacrifices are invariably offered to Agni and through him to other gods. Agni is thus the primary recipient of all Vedic sacrifices.So he is also Divine messenger of the gods. You can read more about Agni in this Answer.
The Rigveda itself indicates that Truth is one - "ekam sad vipra ...
Besides Shree Shankara Shree Madhvacharyas commentary on Kena-upanishad in english is available for PDF download .
Here is Shree Madhvacharyas Bhashya on Kena Upanishad - Translated by Nagesh D. Sonde.