Although these engines are little high performance with precision made expensive monsters still things are done to lower production costs. The heads we see on all engines are easy to produce on not such expensive machines. The fastest way is to mount several groove bits side by side on the lathe and turn all grooves at once in one move on a bar of aluminium.

Yes, they do, you can also think of a one piece head with combustion chamber which do exist. First of all as an old IT hardware service engineer I know the struggle of cooling paste getting everywhere than only on the cooling head. It makes engine maintenance and checks a dirty business ;)
But I also believe some heat resistance between button head and cooling head acts like a PID controller to keep the temperature within the sleeve more constant. Yes, I have heard people use it, a more simple and cleaner trick some people use is to put a small layer of oil onto the button head that will acts like a little bit the same. And with some cheaper models they leave the anodization on the head that is is in contact with the button head, that for sure is no good.

https://cimg4.ibsrv.net/gimg/www.rct...7050f758dd.jpg

This is one of the 1st SEG 21 engines which was GO made. I did remove the layer. See: SEG21 onroad | ModelbouwForum.nl