It seems as though you want to be told that you would do well in the program. Only your mom will tell you that (no offense intended).
I know people that have done poorly in high school and college that have succeeded in the program so far (I am nearly qualified at prototype). For example, my power school honor man had a GED. In my experiences, I would say that the program will not be easy if you have high grades nor hard if you have low grades.
More to what I perceive is the root of the problem: you will personally learn a lot of completely new material, you will personally put in a lot of time at work, you will personally be challenged by new concepts, you will personally LEARN these new concepts. No one else will do any of this for you. People will definitely be there to help you along the way, but you will not make it to the fleet because you are "a smart guy". Excuses and reasons for not completing tasks will be met with punishment and possible masting.
More specific to your situation; you're grades are essentially unimportant. Your drive and ability are paramount. I hope that you have somehow felt compelled to share your grades [through peer pressure (many others do it)] and subsequently shared your reasons for low marks to save face. If I were you, I would do this all over again except I would say that "I am looking for an opinion about going enlisted and getting my degree while active duty vs. graduating college and going officer afterwards".
Hopefully you are just nervous about entering a community populated by typically intelligent people and are looking to compensate for you 2.5 marks (your situation truly does sound difficult, no shame there).
Advice for you: there are 3 ways (as of 5 months ago) to become an officer once becoming an enlisted nuke. First, get picked up to go to the Naval Academy (the selection rate is extremely high once making it to SC). Second, get picked up for STA-21 (much more competition). Third, get picked for OCS (only viable if you already have a 4-year degree).
Personally, I am not qualified, I am not on a ship/boat, I have been in for 1 year, 11 months. I HAVE recently gone through (going through) the program that you are worried about entering. Take my advice as you will.