is there a need to create attribute in COUNTRY e.g. "CountryName" & "link" it back to the CUSTOMER ?
If by link, you mean a shortcut, the answer is no -- that is not required. You may want to though if you wish to use CountryName in Customer queries, etc.
1) What do i need to do to prevent the deletion of a COUNTRY if it already exist in either CUSTOMER or SUPPLIER ?
Put a rule like the one below in the Country BO "Rules when object is deleted":
If EXISTS Customer WHERE (Customer.Country=Country) Then
REPORT ERROR 'Delete not permitted. Country exists in a Customer'
2) What do i need to do if i allow a COUNTRY to be deleted even if it already exist in either CUSTOMER or SUPPLIER ?
AwareIM will allow you to do this if it is a peer relationship. You do not have to do anything special.
Tom