6000 words, with corrections and the name of the error.
No. I won't make any content changes or write anything myself. The most I'll do is point out errors and give a short lesson on how to fix it for the future; the onus is on the submitter to fix it. If you're still unsure, please ask or send this to the relevant authority (eg the course leader).
Yes. As long as it's under 6000 words, I'll happily proofread it for you.
Of course. As a default, I'll provide comments in bolded red text, with the original intact for comparison. However, if you'd prefer another method, let me know.