Thanks! I think I can now apprehend your point of view (namely, you were saying that Queen Elizabeth's letters patent in 2012 made a general rule for children of the eldest son of the Prince of Wales, but left it to future monarchs to sort out the situation of children of the heiress apparent of the Prince of Wales, or children of the heir/heiress apparent of the heiress apparent, if and when one of these situations arise).
However, I would say that the letters patent of 2012 also made/confirmed the "general rule" that children of an heiress apparent of the Prince of Wales (or children of the heir/heiress apparent of the heiress apparent) will not be Prince/Princess. Even if the 2012 LPs do not explicitly declare it, the fact remains that they were not made eligible under the 2012 LPs and therefore, under the existing framework of the letters patent of 1917, they are ineligible to be Prince/Princess.
A future monarch could certainly issue new letters patent to change this, but a future monarch could also issue new letters patent to change the rule for the children of the eldest son of the Prince of Wales. As such, I cannot see a reason why only one of these rules should be seen as a general rule. If the fact that children of the eldest son of the Prince of Wales will be Prince/Princess for the time being is a general rule, then (in my opinion) the fact that children of an heiress apparent will not be Prince/Princess for the time being is also a general rule.
I agree with your second part; limiting the letters patent to the Cambridge children would not have achieved that. But it would at least have avoided the making of a different general rule for children of an heiress apparent than for children of the eldest son of the Prince of Wales.
It is also worth mentioning that under the letters patent, the general rule for children of eldest sons applies even when the eldest son of the Prince of Wales is not his heir apparent. Had Charlotte been born first, George would have ceased to be in the direct line of succession in 2015 when the Succession to the Crown Act came into effect, but under the current rules, his children would still be Prince/ss from birth, even if born in Charles' lifetime (while Charlotte's children would become Prince/ss only when their mother became Queen).