We know Tsun protects the bridge leading to sovngarde's main hall of heroic nords. so he is called the God of Trials against Adversity.
But Bosmer also has a god called Z'en, and is the God of Toil. Is it far fetched that they are related and the same god in different provinces?

One in Valenwood, the other in Skyrim.

They're both variants on Zenithar, relating to his sphere of work and labor rather than his economic elements.

So is it fair to say...

Imperials worship Zenithar
Nords worship Tsun
Bosmer worship Z'en

and they are the same god?

Yes and no. Those are all aspects (made real by mortal belief) of one of the Aedric planets (which are really the dead bodies of the "true" gods).

Do you know about http://www.imperial-library.info/? You can find the answers to most of your questions there.

"Shor Son of Shor" connects Tsun to Trinimac. Whether he is also Zenithar, and what that means for Malacath are different questions entirely.

It's more like Zenithar is the Tsun is to Z'en as Akatosh is Alduin is to Auri-el.

They are all different gods, despite all being based off of the archetype of "God of Labor".

Check out Tsun's buckle, and check out Malacath's buckle in Skyrim. They mirror one another. Tsun's is that of a demonic head, while Malacath's buckle is that of a skull.

Trinimac was demonized, hence the demonic head. Tsun was slain, hence the skull.

I don't think it's coincidence.

Everybody is Trinimac. Trinimac is the freaking village bicycle of being Trinimac.