Zest shouldn’t, if the recipe calls for it.
The lem lux is the TCB Golden Luxury Butter Cake, one of my faves, with lemony additions. I don’t think there is a wedding cake for it, specifically, so I can’t direct you to the wedding cake section for leavening.
One 9x2 (vol = 127 in3) round makes a little more than two 6x2 rounds (2 = 113.04), so you should. I think lem luxe calls for two 9x2 layers, so 1/2 of that recipe should fill your two 6x2 pans with some leftover. But, as you say, you’re filling the pans 1/3 full, so you should be safe in any event.
As you say, you decrease for larger cakes and increase for smaller. However, leaving it the same (rather than increasing BP slightly) for a 6x2 should work, with the worst to happen being a domed (rather than flat) layer.
Geez, I am perplexed!!!
You aren’t weighing the leavning, are you? It’s best to weigh everything except small quantities, like leavening, salt, spices, etc., because scales aren’t accurate at such minor quantities—you could be over-leavening if you are weighing those instead of measuring. Is that possible????