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Well the SYI led to the blow up between the 2 characters concerned.....but I guess there is a fair case too for NM as the period to be highlighted.
I still maintain however that the phrase 'Correct implementation' means you have to do something in the grid after it has been filled to go honour the theme. If this is to lift the girls skirt (whoops!) then the last letter of the period disappears.
Ambiguity rules OK!

First post here so apologies if I say something wrong.
If I write both at the first and last letters of my extra words from the across clues into the central column I can find a phrase reading from top to bottom using one or other letter of each pair. If I choose the other letters from each pair then I see exactly the same phrase from bottom to top (with an appropriate letter written into the bottom row.) So it seems the decision is not to reverse the top to bottom phrase but which version to pick in the first place. Something that has not yet been b???? u? or something that is already b???? u?.

Great spot Lilith!
Seems then that the central phrase must logically be entered bottom to top which means that the NM period is short of its last letter. Must be SY then that is the period to be highlighted.

I'm almost convinced. There's just this little voice at the back of my mind telling me that the LH period is so well-known and the RH one sufficiently obscure that I wonder why the RH one is there if it isn't going to be used. That's me trying to get inside the setter's head -- backto "guess what I'm thinking"!

I'm tidying up and noticed that I have one incomplete answer. It's 2 Down...

Abolished name adopted by Great Britain and N Ireland (3) UN?

There's a S ignored by wordplay.

sunk ....the s goes at the bottom
UK (G B & N I) with n(name) inside

Thanks, Kirky - I'd tried various options, but that wasn't one of them!