You seem to be a GREP specialist, so I guess I won't be able to bring up directions you wouldn't be aware of...
Actually, I don't understand why you seem reluctant to Option 3... How come updating the index could be a problem?
Assuming you have a char style applied on first names and another one for last names, I would go for Option 3:
<Idx:=<IdxEnType:IdxPgEn><IdxEnRngType:kCurrentPage><IdxEnDispStr:$6><IdxEnDispStr:$2>><cs tyle:firstname>$2<cstyle:> <cstyle:lastname>$6<cstyle:>
Obviously, that would work only for pattern "first name" + "any space" + "last name"... but you know that...
4th option: the best approach to make Indesign index sub-entries I've read has been written a long time ago (2009) by Guy Burns.
[ After fixing bugs, tested at the time with InDesign CS4! Great! ]
@Obi-wan Kenobi - Is this the article you mean? http://www.kahrel.plus.com/indesign/index_edit_topics.pdf Could be promising. Will try it out soon and report back.
@vinny38 - there isn't a reluctance by me to do it. The names are in one style only - given names and surnames, but it is still possible to GREP this solution... at either stage (e.g. make one style TWO while in indesign, OR let the GREP in Textwrangler differentiate between the first words and last whole word). This solution will work in my instance if the text is in one continuous story... if the text was in several stories (or hundreds or thousands) then this method wouldn't be as appropriate, and I'd try Obi's method above... though that will still require the index entries to be listed using the shortcut shift+option+command+] . Obi's method in this instance will require a one-off roll-through using find/change to find each character style and then apply the shortcut, but on 700 entries at arguably 5 seconds an entry, will take just under an hour.
Looks like the answer lies in one of Peter Kahrel's scripts that Obi-Wan Kenobi was referring to: http://www.kahrel.plus.com/indesign/index_edit_topics.html
It also answers my question a little better than my third suggestion of saving out an indesign tagged text format, given that my brief has changed - instead of being one frame with index entries highlighted in a character style, it is in fact hundreds of text frames with TEN different PARAGRAPH styles assigned... so my workload just went up with or without a script!
I have formulated a way to do this but it is a single serving purpose and will just bore readers to tears, so not worth posting here.