How to protect the timeline of my project when the supervisor is not willing to negotiate anything?
You are still 7 months off. Work hard, but don't overwork yourself. 3 months before the deadline, re-evaluate where you are are: either you will catch up, or more likely, you will fall even further behind. If first, great, you win, if second, you now have a much better leverage to force descope of the project. Go to your advisor and say: ok I've been working really hard for 5 months, this is what we got, this is what we didn't and it was due to these unforeseen circumstances. Then make it absolutely clear that you plan to leave after 3 more months due to a job offered and that it is hard to buy food with no salary. She will probably huff and puff, whatever, at worst your grade won't be perfect, but then again, if you have a job lined up, it shouldn't matter. Whatever you plan to do after that job will be judged more on that job than on your (possibly) mediocre grade.