Regarding Tsun, Varieties doesn't explicitly mention him being a precursor for any of the divines (unless I missed it). If you have a source that indicates otherwise I'd be happy to look at it.
Ay, you know as well as I do that these things are often hinted at rather than spelled out explicitly.
In this case it's inference, reading into the similarity of the name 'Tsun' to Z'en, Xen and Zenithar, the similarities of these gods' spheres (Tsun's 'trials against adversity' correlates to Z'en's 'toil' and Zenithar's gentrified sphere of labour, craft and commerce - and this is in keeping with the visible trend that the spheres of Nordic gods tend to be rougher, harsher versions of their Merish and Imperial forms), and the similarities to Stuhn->Stendarr and Jhunal->Julianos which are known transitions from the Nordic pantheon to the Divines. Stuhn->Stendarr and Tsun->Zenithar even share similar sound changes.
Put it this way - I'd eat my Colovian Fur Hat if Tsun were just a completely isolated deity rather than an old aspect of Zenithar.