Learn English – Where should we use ‘has/have been’ and ‘had been’

I am also British, and I say there is no semantic difference between your answer for question 1 and your teacher's answer. Also, they both have valid grammar and as such they are both correct.

It's possible that in your teacher's variety of English his form is grammatically preferred over yours, but without a wider context to the phase it's hard to say.

The presence of the near demonstrative "this" gives the sense that the speaker is saying it while being in the workplace, perhaps while actually working. In that case, in British English, it sounds a little more awkward (though not incorrect) to use "have had this job". The most usual form in that situation would be "have been working here", but that's further from the original form of question 1, so maybe that's why you're teacher went with "have been in this job"

But really this is splitting hairs and your teacher shouldn't have marked you incorrect for "have had this job"

Modal verb must denotes obligation as its most primary role.

You must finish the project till tomorrow. - It is obligatory for you to finish the project till tomorrow.

Past obligation is usually denoted by the equivalent modal phrase had to.

The task was urgent and I had to finish it as soon as possible.

In both examples, the action expressed by modal verbs will happen after the time of speech.

Now consider the following examples where the modal verbs must id used to denote deduction.

His lights are off. He must be out. - The action denoted by the modal verbs is simultaneous to the time of speech and it deduces new information from the current situation.

Must have been is used to denote the past experience of the same kind.

You must have worked a lot to be such a successful scientist.