I have a few suggestions.
First, coconut pairs well with lemon taste, so coconut flour is another primal option. You could also dust in coconut rather than more sugar.
Second, whipping egg whites with a bit of the lemon juice will stabilise the egg foam. But nonetheless, you want to do this step last (just before you fold them into the batter) to maximise the amount of air in the mix.
Third, I'd whip the sugar into with the egg yolks until lighter in colour, and then add the rest of the liquids. Sugar is normally a 'wet' ingredient in batters, because it traps air bubbles in fats like butter and egg yolk, leading to a spongier texture.
Fourth, as this cake is leavened by the egg whites, you should let it cool to room temperature while in the mold, upside down (I doubt they'll fall out of the ramekins). This will get gravity working on your side as the cake firms.
Finally, give it a practice run before you need it, just in case something is slightly off. You have to change a lot to make it primal, it could do things to do cake.
"Thanks to the combination of meat, calcium-rich leaf foods, and a vigorous life, the early hunter-gatherers were robust, with strong skeletons, jaws, and teeth." - Harold McGee, On Food And Cooking: The Science and Lore of the Kitchen