Depender en is not a grammatical phrase. Depender goes with de. What you have found are instances of depender de in which other phrases come in between depender and de. This is perfectly correct and very common. Depender de is not an inseparable fixed phrase.
In all your examples with depender en, the preposition en is part of a complement that you could in principle delete...
Por tanto, estamos empezando a preocuparnos, una vez más, por depender en mayor medida de Rusia en el futuro.
or move:
Por tanto, estamos empezando a preocuparnos, una vez más, por depender de Rusia en mayor medida en el futuro.
In this other example not one but two things come between depender and de (marked with [square brackets]): a complement of time (en algún momento) and the subject (su adhesión):
¿Va a depender [en algún momento] [su adhesión] del veto de alguno de los países que ahora son candidatos?
As above, you could move things around so that depender is followed immediately by de(l):
¿Su decisión va en algún momento a depender del veto de alguno de los países que ahora son candidatos?
As you see, Spanish allows for a lot of movement within the sentence.
Why do these things come in between the verb depender and its dependent phrase headed by de? Well, because they can, and because it sounds better or more natural sometimes. You could have something like
Dependemos para lograr este objetivo, a causa de la situación actual, en gran medida y por un plazo de tiempo indefinido, de...
where the verb and de are separated by several long complements.
