How do you strengthen plywood?
You could laminate it with fiberglass weave with a strong adhesive (polyester).
What is the difference between marine grade plywood and treated and will paint stick to treated plywood?
Marine grade plywood and treated plywood aren't the same thing. Marine grade plywood has special glues and woods that will withstand constant immersion in water. Treated plywood is regular plywood that has been run through a pressure treating cycle. Paint will stick to treated plywood if you make sure to let it dry for a few months before you paint.
Any strong wood. In the US, softwood plywood is generally made from Douglas fir or southern yellow pine. Most hardwood plywood is "veneer core." This means they make sheets of plywood from poplar--an inexpensive hardwood but plenty good for plywood--then put veneers of a more expensive hardwood on both sides of the finished sheet. If you run into oak "marine plywood" all the plies are made from oak. Oak is extremely hard to make into…
¼-inch plywood 0.31 3/8 inch plywood 0.47 ½-inch plywood 0.62 5/8 inch plywood 0.78 This is taken from http://www.sizes.com/units/rvalue.htm, which also has some other materials that could/would commonly be used alongside with plywood to increase the R Value such as building paper and insulation... if both sides of the frame are ply then you must also include the void.