First, while I do not agree with everything Fr. Chad says, his advice is generally good.
For your point about NFP, it appears that Pope Pius XII uses “grave” and “serious” interchangeably… refer to the capitalized words in this authoritative text, Address to Midwives, from Pope Pius XII.
“SERIOUS motives, such as those which not rarely arise from medical, eugenic, economic and social so-called “indications,” may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life… If, however, according to a reasonable and equitable judgment, there are no such GRAVE reasons either personal or deriving from exterior circumstances, the will to avoid the fecundity of their union, while continuing to satisfy to tile full their sensuality, can only be the result of a false appreciation of life and of motives foreign to sound ethical principles.”
However, the Catechism moved away from using serious, to “just,” “serious,” “proportionally serious,” and in Humanae Vitae says “well grounded” and “reasonable” motives. For this reason, the Church has definitely moved away from Pope Pius XII’s language of grave. And the Holy See does not appear to be headed back to that language. So “serious” and “very solid” is the most probable opinion.
However my point remains: do you have specific evidence of a different meaning (before 1900) when “grave” and “serious” were used? Given so many new “changes” are indeed in contradiction to some teaching somewhere, the conservatives are typically right, if you accept everything in the Magisterium. But I won’t form a strong opinion on this until I study this more. Let me know your thoughts?