All the combinations extracted from both of these,
- "Have you been influenced (in) all (that) you can ever be influenced?",
- "Have you been influenced (in) all where you can ever be influenced?",
are awkward sentences, at best. It seems to me that the way out of such an awkward phrasing is to change it altogether.
Most likely, "all you can ever be influenced" has to be taken as meaning "all the influence that can be brought to bear upon you". I'd therefore write this.
- Have you been subjected to all the influence that can be brought to bear upon you?
In any case, this is a consideration of a rather philosophical nature where the fundamental basis is uncertain: the premiss of a maximum of influence as determined for a given individual does not really exist; this maximum is defined for nobody, nobody has a true idea of what could be this maximum influence, no idea of what context among some possible ones could be propitious to the materialization of a maximum level of influence. This makes it an unlikely sentence.
Addition due to a comment from user Sssamy, repeated here for convenience
I think your first example resembles this 'influence' construct in that 'all' is not directly an object, but probably called an adverbial object nonetheless. I came up with some sentences. I gather my first would pass as fine but the rest would not. What do you think? (a) You probably need the same verb before and after 'all.' (b) Having 'can' after 'all' is a plus. (c) The latter clause should be complete (= it can stand alone). (1) You will be helped all you can be helped. (2) People will help you all they can help you. (3) You will be helped all you try to achieve. (4) You will be helped all you try to achieve a goal.
This turn is strange to me, I haven't read nor heard anyone use it, ever; as I read it I don't identify rapidly what is being said and it is only after reflection that I can give it some sense. It sounds a little as sounds the idiomatic turn in "I all but fell.", and I can conceive it as such, just an idiomatic turn, albeit not in use and yet to be forged into the language. In fact, your prescription for the grammar of this turn is typical of that which the nature of idiomaticity forces upon the grammarian's reflection: terms that are necessary, that are also unchangeable, there being in the structure in the end but few things that can be used according to the usual variability that composability and grammar endows them with; that is idiomaticity in essence, there is no grammar, no grammatical relations on which to fall back. Short of this idiomatic point of view, I persist then in looking on this construction as ungrammatical.
I truly don't see how the two verbs being the same could contribute to the grammatical relations nor which relation in particular. Why not consider imposing this restriction for the subject (which is the same in your initial example)? This is just as arbitrary; such criteria are not used by grammarians or if they are the occasions must be very rare; I've never heard of those principles. I am curious to know what would be your rendering of these sentences in a phrasing into which can be clearly read the grammatical relations.