Have fun with your reupholstery!
Answering your questions and concerns, I like the suggestion of using some firm latex foam topped with a softer layer. I think that will be very comfortable.
Many glider chairs would not have springs, so this is not an unusual configuration.
If the geometry of the chair is right for you - meaning that the seat is the right width and depth, and the back hits your back at a comfortable angle, then the chair will be comfortable with a foam padding.
I wanted to give you an accurate answer on how thick the batting should be, so I called Sarah Hardy over at Michael's Custom Built Upholstery. They are expert in green upholstery, and we had a nice discussion about your options.
- The batting layer outside of the foam is there to add softness and curve to the finished cushion.
- The curve of a cushion above flat is called the crown, and more traditional pieces tend to have more of a crown, and a softer overall cushion.
- The more tailored mid-century sofas often would just have latex foam block with no batting, for a flatter cushion with no crown.
Therefore, the thickness of batting you use is really a choice. I think I would suggest that you use over an inch, and perhaps a two inch thickness would be most appropriate for a glider.
Finally, adding a layer of ticking over your wool is a good idea. Wool is not as dimensionally stable as polyester batting, it tends to stretch and move around. The ticking will help it stay in place. However, you do not need a down proof barrier cloth. Any tightly woven, high thread count muslin or sateen will work.
Hope this helps you with your project.