Rather than having a separate birthday cake, how about having half the cupcakes as naming day ones, and the other half as bright colourful birthday ones?

For the naming day I think it is nice if the mentors speak a few words, like a pre-prepared poem, something that reflects how they will guide Miss S through her life and support her. Or perhaps each person could say something briefly as they release their balloon?
We have a promise like thing for each of the mentors (a bit like the I - do at a wedding )

For our kids we've had candles. Each of the godparents has one of their own and they use it to little the LOs candle (ours were decorative and kept as keepsakes)....will have al ook and see if they little poems for you if you like.
My sister had coloured sand. Each godparent picked a coloured sand and during the ceremony they had a bit where they all poured some into a bowl and all had a stir and made a wish. Then my sister poured it into a decorative jar to keep. You could even have them write their wish on a piece of paper and put that in the jar with the sand.

We had DS ND and DH b'day together. I did sepearate invites in the one envelope. I also made separate cakes as i felt they were separate occasions (but don't do what I did - I made DS a Noah's Ark cake, complete with handmade animals, and DH a 3D dragon cake complete with fire- I was finishing them as they were being brought out lol).

We did DS's naming day first and then his little celebration and then moved onto DHs birthday so they we still separate but together IYKWIM?
I was thinking sepearate invites in one envelope and doing the naming day stuff first too. And def no making intricate cakes like that LOL