A picture would sure be helpful on this one and ansewer a lot of questions.
What you have sounds like what's called a shed style roof. There very prone to leaks because no one ever gives them enough slope, and do not tie them into the existing roof correctly.
That narrow a 2X on that long a run is undersized and will cause the roof to sag.
The fact you have sky lights adds to the chances of leaks if there not low slope lights.
Is there any drip cap around the outside edges? (most often an aluminum strip of metal bent at a 90 deg. angle to protect the outside edges of the sheathing)
Are these 3 tab shingles? There the cheapest one, most likly to leak and have the shortest warrentys in the business.
Some other choises for low sloped roofs are EPDM, it a one piece rubber membrain. They can come in white and black. In your area a white roof would stay far cooler.
Metal, can come in many colors, and some have warrentys in the 75 year range. (not the cheap junk sold in the box stores)
If you want to hide the under side of the rafters there's lots of options.
I use vinyl soffit and vinyl cove moulding made by Cereented. The best looking one is called beaded. In your case I would use what's called hidden vent. When it's installed you can not see the vents and it will look like you installed Tongued and groved boards. Any real siding supply house will have it.
I just hate to see someone try and use paneling on the ceiling. Over time it will sag and there's batten strips to hide the joints.
Real wood T&G will be a nightmare to keep painted and will give you no ventalation to keep the bottom side of the sheathing dry.
PS Far be it from me to correct anyone spelling but the other poster misspelled Advantec, there is no K on the end.
It's much stronger then plywood, comes tongued and groved for butt joint strength, and holds up far better when wet. No H clips needed.