Just to expand things a bit .....
(Information taken from Shee Twohig's <i>The Megalithic Art of Western Europe</i> - a book which is better at descriptions than explanations! The drawings in it are taken from rubbings done in the late 70s so may show more than currently visible.)
At <b>Kerguntuil<b/> there are on one stone the the six pairs of "breasts" with the "necklaces" below five of them as shown in your post, Moth. Here the "necklaces" are single, unbroken U-shaped lines
On the stone to the left of that one, are another two pairs of "breasts", one pair on its own and the other with a "necklace" below it but here the "necklace" is a more of a dotted line - like a set of beads if you want to pursue the necklace idea. This second one is almost exactly replicated at <b>Prajou Menhir</b>.
At <b>Tresse</b>, there are two sets of four "breasts" with a "necklace" of the unbroken U-type below each of the right hand pairs.
While the relative positions do not support the necklace idea, you could argue that the "pregnant belly" theory works well with the unbroken line but not with the dotted line.
In the Paris Basin, the "necklaces" are all above the "breasts". There are no groups of "breasts" , just pairs. As for the necklaces, no single, unbroken U's here; they are either three or four unbroken lines of nested U's or one or two lines of "beads". (Diagrams would help here but nowhere to post them!)
At <b>Trau-Aux-Anglais</b>, there is a suggestion of a face above the necklace but it is faint and the proportions are all wrong.
Just to add to the connections between the Breton and Paris art, the rivetted square "shield" seen at <b> Prajou Menhir</b> in Brittany is also seen at <b>Missisippi</b> in the Paris Basin so maybe not so easy to dismiss the two as separate traditions (not saying you were, Moth).
If you want support for the "breasts" and "necklace" idea (to answer your original question) ie that they are not abstract designs wrongly interpreted by archaeos, the carved figures in the Paris Hypogees include two with faces, necklace and breasts all in the correct places! (Another one has a face and necklace without breasts although it is damaged). One of the Gran'Mere statue menhirs in Guernsey has a similar stylised design.
So where does that leave us? There seems to be enough evidence to say that the designs in the Paris area most likely represent breasts and necklaces. I think that explains why archaeos use the same terminology for the Breton sites even if it makes no sense in terms of their positioning.
Its not enough to say, as Twohig does, "when necklaces occur in Breton gallery graves they are inevitably found below the breasts <i> as if their significance was not apparent to the carvers</i>" . (My italics).
I don't have an answer to the Breton question but I'm not convinced about the "pregnant belly" idea because of the dotted line versions.
Phew! Sorry about the length. Fascinating subject though.