I can't think what that means! There is 'the Emerald Isle' for Ireland, but apart from that...
y Wlad Ddu might possibly work for 'the Black Country', which is part of the West Midlands area of England where there used to be a lots of heavy industry and coal-mining. Really, though, y Fro Ddu might be better for that since there is no actual country/state involved.
Not necessarily in this case. Sometimes, w acts as a consonant rather than as a vowel, and this can often be the case with words which begin gwl-, gwn-, gwr-. When they are mutated following y, the y often stays as y rather than changing to yr. It can come down to personal preference and habit, though.