Two 4GB DDR3 modules and two 8GB DDR3 modules.
Two 4GB DDR3 modules and two 8GB DDR3 modules.
Yes, your motherboard can support both 1333 and 1600 MHz DDR3 modules.
I assembled a setup using a Z97-A and it handles 2x4GB at 1600 MHz and 2x1GB at 1066 MHz, suggesting it might just adapt to the available JEDEC specifications.
That should function properly. Simply insert the additional RAM.
Read the speed from the chip profiles, not the real-time speed. Adjust the BIOS speed and it will update.