Career wise, what's your plan? What job do you want to get with your ID degree? As far as I can tell, getting a job in ID is much more about your portfolio than degrees. Knowing people in the industry will also get you a long way.
You're in a weird situation in that if you go back to school to do 4 years in ID, you wouldn't be at full course load. I'm guessing you wouldn't need the liberal art credits and you could probably be credited a some technical classes from your engineering background. Obviously, it would be pretty great if you could work part time at an ID gig during that time and finish with a super strong portfolio and even have some professional experience.
I'm not sure where your skills are currently at in ID but would a masters in ID be a possibility?
As far as the online courses are concerned, I think you would loose a lot of the interaction with a class and professor. On the other hand if you want to start building the skills to put together a portfolio, you might as well do it with an online class and have that be recognized. If you can shave a year off your financial burden by doing the classes online and work on projects on your free time, you might come out ahead. A lot of first year seems to be sketching, form and CAD basics. If you start sketching now at 10hr a week for two years while you rake up the dough to go to school, you'll be miles a head of doing it a for a semester, weather you're in class, online or on your own...
In terms of online classes, have you gotten in contact with SCAD? I don't know where you're located in South Carolina, but it may be possible for you to travel a few times a semester. It doesn't seem like they offer ID as an online class but have a whole bunch of closely related programs available. Maybe you could come up with some kind of arrangement with them.
Also, have you considered studying abroad, at least for your foundation classes? The cost of education in the US is really quite amazing. I mean, a full year at Art Centre is more than I paid for my 4 years of mechanical engineering at a reputable Canadian school.
TL;DR, look at ways to get the skills more than getting a piece of paper.