"1) Domia X10 - is this the same as the new devices from http://x10-hk.com?"
Don't know but they Certainly look similar
"2) Is Domia X10 compatible entirely with normal X10?"
Yes
"3) The Domia devices apparently contain filtering - does this help with the reliability issues? Do you only get the advantage of the filtering if you use all Domia stuff?"
I have a mixture of modules, in my house, from various suppliers we use. Some have filtering and some don't. It all works fine.
"4) Is there anyway to interface Domia X10 with Domia Lite? Is there any point?"
Why would you want to do this. I think Domia lite is wireless.