You'll have to have your commercial certificate first, but there's no reason you can't do your CFII as your initial flight instructor rating. It will probably take a bit longer than it would if it were an add-on because you'll have to become familiar with the Fundamentals of Instruction as well as the IFR material. The CFII written is from the exact same question pool as the IFR, so if you didn't already take it, it shouldn't be too difficult for you to refresh you memory on the stuff.
From where it looks like you're at now, you'll need to do this:
Take the Commercial written
Do the Commercial training and practical test
Take the Fundamentals of Instruction (FOI) written
Take the CFII written
Do the flight and ground training for the CFII
Take the CFII practical test
At 4-6 hours per week you're probably looking at 3-4 months for your commercial cert. since you'll need 250 TT before you can take the practical (unless you're part 141, then the time restriction is less). After that I'd give it another 3-4 months since it is your initial flight instructor rating and it takes a little extra ground work.
I only flew 1-2 times per week and my initial CFI (I did it the traditional way and went CFI then CFII) took me about 9 months to complete. Of course I was also flying for fun in between so it stretched it out some. I added on my CFII in about 4 months, squeezing in about 6 flight lessons and 6 ground lessons in between teaching students and working a 40 hr/wk day job.
You seem to have more time and resources to dedicate to it, so there's no reason you couldn't complete them in about 1/2 the time I did.
Good luck!
Carolyn