Question: Can the impact character forcing the protagonist to question their habitual ways of solving problems be a sort of 'generalised other'? Eg an environment the protagonist finds herself in, where everybody else seems to be doing something a different way?
If so could this be exemplified through various different characters who all take the same approach but different to the protagonists?
Answer: Great question. Sounds like what you are describing is a "man vs. society" type of conflict where the main character finds himself at odds with society as a whole, where his dilemma is whether to conform to social norms or rebel.
Certainly, you can make society itself the impact character, though it is more typical to have a character who represents the attitude or approach taken by society (e.g. Number 2 in the television series The Prisoner or the Prime Minister in A Clockwork Orange).
It is also perfectly acceptable for the role of impact character to pass, like a baton, from one character to another. The main character could encounter several people at different points in the story who have the same attitude/approach and who challenge him in similar ways.
Another variation is stories that have two impact characters advocating opposite approaches. The main character in such stories is often very innocent or naive. He has no experience handling the type of problem before him, so he must choose which of the two impact characters to emulate.
Note that the impact character doesn't have to actually appear in the story, as long as his influence is felt. You could have a character like Big Brother in 1984 who is a mere symbol representing a particular attitude/approach taken by society at large.
Another sidebar: It is common in this kind of story for the impact character to be the antagonist as well, though this is not necessary. You could have a story in which society faces a particular villain and the main character must choose between society's way of tackling the problem or his own.
Lots of possibilities, but the short answer to your question is yes.