HI Buddy,
Two layers of 5 alone should be enough cloth. I actually use 6 oz cloth, but I feel 5 oz will be as good here. If I was actually going to apply another light layer of cloth to it, I would wait until I had it off the plug and would be using that layer to seam the tank
After you have laid up your cloth, and the resin has cured on your final layer, give it another coat or resin to fill the grain of the cloth more so you are sanding resin to get it smooth instead of getting into the cloth.
You will never get the tank off the plug if you cut it vertically. To get this tank off the plug, I would cut it horizontal on the side where the wing meets it and across the front only as far as the rounded area. From there you will be able to get water in it to melt the PVA and pop it off. Sliding a very thin ruler or some metal object into the cut line will help the water get to the PVA.
Be sure you have the plug well prepared for the glassing. I use "Meguiar's" #8 maximum mold release wax. I apply a coat, let it dry, polish it off, and then add two more the same way. I then apply a coat of "PVA Mold Release" which is a water soluble liquid mold release.
This PVA is a MUST DO. I assure you that the mold release wax alone on this green plug will not be enough. It takes three or four glassing on a plug or a mold to actually get it cured enough so wax will be enough.
Also I am assuming this is primer I see on your plug. I hope it under your glass. I actually tried to paint a plug once thinking it would make it real smooth. It was a disaster. When this resin cures like any resin, it generates heat. This heat melted the paint into the PVA and I just about destroyed the plug getting the glassed fuselage off of it. I figure it would do the same to primer.
Yes you are definitely going to have some overlap area on that nose. I would start my sanding there. Probably using 50 grit paper to cut it and then finishing it off with a smoother grit. If I am not happy with the finish on this nose, I would apply a coat of thin CA to again seal the cloth and finish it off with some 150 grit.
Remember the strength is in the whole. I am betting you will find some small holes in the glass of your finished tanks especially in the nose area. Simply back these holes up with a piece of wax paper and fill them with baking soda and thin CA. You can do this before you seam it.
Being this is just a wing tank, I would just seam it using thin CA. Once I have it glued together. I would try poring some resin through the hole in the back of your tank along the seams.
All this sounds sort of complicated. It actually isn't once you have all the right products and start the job.