This needs to be clarified...to actually get a flight slot without getting accepted into a regular commissioning program is very rare, although the navy is running a test program which does exactly that (I don't think they are taking any more applicants though). The navy also did this during WW-II, not that it helps you any.

The army is the exception, you can into army flying as a warrant officer, and it's a little bit easier than some other programs. However...I cannot guarantee that a 4-year degree with a low GPA would beat out a 2-year degree with a high GPA in the army. They are still looking for quality folks, they just don't put as much emphasis on higher education.

If you are APPLYING for a commission (aviation guarantee or otherwise) enlisted service will help. It shows that you can handle military service and do not have unrealistic expectations or attitudes about what you are getting into. Also most military folks do like to take care of their own. But be aware you need to excel while enlisted. Mediocre performance will do more harm than not enlisting in the first place.

But since you have the degree, try to get a commission first. Make sure you explain your schools grading standards in your letter. I suspect they will fall back on your class rank to get an idea of where you stand. It will help if you had a difficult major (engineering/physical science). If you were in the top 20% of engineering grads, let them know that and try to document it somehow.