Your questions, Denise ...
1. The short answer is 'very strict' - there is no margin, and since the WA is submitted digitally, it is very easy for examiners to check (they will not read beyond the limit, which is likely to damage significantly the effect of the text). Sorry, but your student is going to HAVE to shorten.
2. I would be doubtful about this, Denise - it's an interesting idea, but I feel that the point of the WA is that it should be a dedicated, original piece of writing, not a 'copy & paste' from some other piece of text. I can't say definitively without seeing what the student proposes ... but I think it would be a very risky project (and anyway, would need justifying very well in the Rationale). Surely there will be a 'creative' way that the student can use the diary material ...?
This seems a bit tricky, Selda, and you're right to be cautious. I don't know the prose poem by Kincaid, but I can only imagine that the whole approach is fundamentally different, in technical/literary terms, to 'The Bell Jar'. This could possibly be useful ... but only if the style/approach of each had some direct relevance to the research question. I presume that the research question must have something to do with the themes & message of each work - since what else would two such radically different genres have in common? Does each work have something distinctive to say about the protagonist? Or about human nature? Or what?
I think the IB would accept in principle the comparing/contrasting of two different genres ... but only if, in practice, there is some credible argument for doing so. I would require the student to (1) explain clearly what point she is trying to make, and then (2) produce a very clear and explicit research question addressing the point.