Learn English – the correct position of the preposition “before” in the sentence

You got both right. "Ever" modifies "have seen" so it goes before that and follows "not". However, I often see it between "have" and the verb, as in: "Jan can't have ever seen a mobile phone before."

The second is fine. In that sentence, the adverb "undoubtedly" could actually be anywhere and not really change the meaning. It modifies the entire phrase "must have been treated badly" and not just one particular word in that phrase. Usually, adverb placement will change the tone and emphasis based on its placement but that depends on context.

Neither is good. Moreover, "work" and "position" are not well suited to be in the same sentence. It's like saying "I want to work standing on my hands with my head down", or "I want to work bent over".

Replace "position" with "role", and you got something:

I want to work in this role.

Replace "work" with "take" or "apply", and you can keep the "position":

I'd like to apply for this position.
I'd like to take this position.

... although it's a stretch (pun intended).