Question: I am an editor for a trade publication with 40 years of experience in journalism. The company wants me to take a job on the events side, writing content, helping with organization, moderating panels, archiving video. They haven’t called it a promotion but it would clearly be more intense work and involve traveling to conferences every other month. I’d like to accept the offer but I want to negotiate a salary increase after a six-month review. Can I do that upfront or do I wait the six months and in either case, what kind of percentage would be equitable?

Answer: 
You are playing well above your pay grade. Let your manager manage your colleague. An outcome like "the project got finished but wasn't great, one of the interns worked hard but the other was a nightmare" is common as dirt in this industry. 

Do the tasks your manager has assigned to you. When you're out of tasks, go ask for more. When you can't do something because you're waiting on the colleague, let your manager know. When you get something from your colleague and it's unusably bad, ask your manager whether you should fix it up, or bounce it back explaining why you can't use it. Then ask what you should do next given that you can't do what you were planning to do (use the thing from the colleague.)

You seem to think that if you can't fix this other intern (or arrange for a firing) it will reflect poorly on you. Do you really think your manager is so terrible as not to see what you see? You've reported it, shown the code, the manager has already started dealing out tasks and telling the other intern to use source control, and so on. This is all the manager's job, not yours. Relax and do yours, keep your manager informed, and let the manager manage.
* Inform you manager about the Verbatim copies of code from the Internet, and tell that you see a potential legal liability there
* Explicitly state in commit messages which of his commits you fixed
* Your manager listened to you once, so the next time a huge and messy commit comes which breaks something working and could have been caught by the test, you write an email to your coworker with the manager in CC stating that you need he should show you on his machine how he tested the code and how it works.
* When he submitted something which breaks and leaves during the office hours, you go to the manager shortly after he left and ask him to locate your coworker, stating that you would prefer if the coworker helps integrating the code - state that you can do it alone, but it may take more time and state which of your tasks is delayed by this.
* In case this continues, take notes over one week how much work integrating your coworkers mess has caused you, with specific incidents, go to your manager and state that you "can handle it without him"