As far as my simple research goes, you should pick the thickest one. Thick laptops have more room for cooling and will be quieter. Both have the same tdp range (18-25W) so the level of quietness is for the best cooling solution
I can answer some of the questions, but not all.
a. Definitely two 16GB sticks of RAM. With the way ram works the CPU
controller can only support two sticks (dual channel) and if you had four 8GB sticks, the CPU would have to split the two busses into four virtual busses, therefore reducing your speed. But I would definitely get two 16GB sticks rather ...
Your cpu only has a limited number of pci-e lanes available. Sadly I can't find the official documentation but from different forums and other people with these issues it appears that the 3xxx series apu does not support a second PCIex16 slot.
It's difficult to say on basic specifications alone. the idea Irsu85 brought up is a good one, but for all we know the thinner one might use fancy cooling mechanisms like vapor chambers, has more/larger heat pipes or has an aluminum unibody that assists in dissipating more heat.
If you have the make and model. Try looking for video reviews. Often times they ...