You raise good points and I will ask my Bishop more about this. I'm pretty sure he'll disagree with your position, but, he is also 100% anti-Ecumenical.
He taught me how to dismiss ecumenism in one sentence - "What does Ecumenical even mean? It's Greek for, 'Of the Emperor.' Where is the Emperor today?" Which is the point - how can there even be a Ecumenical Patriarch without an Emperor? It's just fantasy and Ecumenism doesn't exist.
As for someone who is Orthodox yet still wants to save his family - why can't he attend Orthodox Liturgy's twice or three times a month, and go to Latin Mass once or twice a month, and simply refrain from praying with them? If this man is intent on saving or improving his home Church, to the best of his ability, what would be wrong with such an action?
You can find a lot more information and sources about this here:
https://www.oodegr.com/english/oiko...Canon 45 of the Holy,, let him be defrocked.”
From the Apostolic Canons:
1.
Canon 10 of the Holy Apostles: "If one who is not in communion
prays together,
even at home, let him be excommunicated”
2.
Canon 11 of the Holy Apostles: "If one who is a priest
prays together with a defrocked priest, let him too be defrocked.”
3.
Canon 45 of the Holy Apostles: "A Bishop, or a Presbyter, or a Deacon that
only prays together with heretics, should be excommunicated;
if he has permitted them to perform anything as Clergymen, let him be defrocked.”
4.
Canon 64 of the Holy Apostles: "If a Clergyman or a Layman should enter a Jewish synagogue, or
pray with heretics, let him be excommunicated and defrocked.”
5.
Canon 71 of the Holy Apostles: "If a Christian should bring oil to a Gentile altar, or to a Jewish synagogue during their feast-days, or light lamps, let him be excommunicated”
One major issue of going to a Roman Catholic Mass if you're Orthodox is that you're validating the activities and claims of a heretical group (ie, a group that teaches lies about God) by your presence. Heresy is evil as it's a lie, and lies come directly from Satan (again, not saying that all RCs or prots are Satanic, just that they've been misled). If an Orthodox person sees you there, he might think "Oh, it's okay to worship with the heretics." It creates grave scandal. More than this, 2000 years of God-illumined saints have UNANIMOUSLY condemned such practices. Your question would be akin to me asking, "What's wrong with me going to a strip club if I do it to keep a relationship with my brother? Obviously, I won't pay for any lap dances".
Funny enough, the Roman Catholic Church also used to condemn such practices until Vatican II. In the writings of Pope Pius XI's Mortalium Animos, it was considered akin to apostasy to pray or worship with heretics. This teaching of Pope Pius XI is considered apart of the "universal ordinary magisterium" of the RCC because it's a repeat of centuries and centuries of RC consistent teaching. It is therefore infallible according to what is defined at Vatican I. Unfortunately for the RCs, teachings on faith and morals in ecumenical councils are also infallible, as they are apart of the "extraordinary magisterium." So you have a real issue with Vatican II (and consistent papal teaching after Vatican II where popes go to mosques and pray with Muslims, etc) completely contradicting infallible teaching that came before.