For WI:
Since athletes from the Caribbean will be representing their countries and not the West Indies (as they are affiliated with the ICC), the winner of a designated qualifying tournament will decide which country gets to compete if the West Indies get a slot.
For Your England/Ireland question:
There is a republic of Ireland which doesn't come under the UK. The Irish team is from Republic of Ireland, so no issues there. I think they would include Scots in GB team if the Scots agree. The similar situation was there in football as well, many times the boards does not comes to any agreements regd allowing players to play for GB.