Yes, for now.
I haven't really thought about it. The original intent was just to redesign the iconic D&D dragons
Good question. I guess I think of them as different species, so they wouldn't be able to breed. However, I did make Shen (the Dragon of Doom) a black dragon / blue dragon hybrid so I guess I think offspring of mixed parentage would be a mixture of the parents.
See my answer above. I could go either way to be honest, but I am thinking they can interbreed, and the result is a mix of the two that is between the two parents in strength.
What do you mean? Fafnir is not a chromatic dragon, it is better described by the 2e Linnorms. I would give them the 'dragon' type in 5e, but I don't think of them as the result of cross breeding of the chromatic dragons. FYI, when I was much younger I had a nile monitor named Fafnir.
I think I answered that above, but IMO a linnorm would breed true with other linnorms, but not with chromatic dragons.