I have a question regarding NEX extension - can I send my ONT to NEX extension address and if I can, will I be able to claim ONG?
At this time you cannot. Initially ONT was a NEP5 prior to their token conversion to their own Block chain.
However, I suspect ONT and their NEP5 equivalents will be a big (arguably bigger) aspect of the NEX environment down the road if they are able to deliver on the enterprise and STO aspect of their ecosystem. I suspect ONT should be easy to integrate into the DEX given its similarities to NEO.