I am not too sure what you mean by Single Field Index. Could you please provide more details on the exact index you created. I am pretty sure I am not the only who do not understand. Because, as far as I understand, the following are already 2 indexes on a single field:
and
Both are single field index because only one field is specified.
Yes because the . is a word separator for text index. That is why when the index is unique you can only have 1 document with the work uk. Which would not make sense for a field named ldhDomain. That is why you do not want the index to be unique.
By 2 indexes I mean
1 - a text index on ldhDomain that is not unique so that you can easily (without regex) search for .uk or .co.uk
2 - a normal (not text) unique index again on ldhDomain to prevent multiple documents with example.co.uk