I have merged pdf files with libre office draw. it's free, open source office suite and is very powerful with a lot of advanced features. in my opinion it's better than Microsoft Office apart from PowerPoint.
Listen. Grades are not everything. You may perform as an average student, but later succeed in life a lot. Believe me, you will need a good physical and mental health for that later, but if you lose it all now, how to live then? Be moderate with your studies and keep balance between studying and rest. And you're gonna be fine.
Hello. I don't think you are going to be punished that hard. Just say you are very sorry:) By the way, this tool https://essaytoolbox.com/grade-calculator must be useful for some students.