Not really an answer is it? I'm sure you're thinking "not an answer you want to hear, but there you have it"

One of the reasons I love you dearly brother!
Answers frequently come to my mind that I refrain from expressing with pen [keyboard] or tongue until I get the "go ahead" from Him. Sometimes, unfortunately, I sometimes express myself openly when God really wanted me to remain silent. He is still working on us.
Churches are closer while others are further away and easily identifiable if scripture and discernment are used. We can talk hypothetical, but when discussing say, Jehovah Witnesses, Mormon's, Universalist, and the beat goes on and on. (Notice I didn't include the RCC. Would have been too easy.) We cannot use their standards to determine viability. Only the Word of God is valid for this. Each of the ones I mentioned, could easily, by overlooking doctrine, be accepted as applying "good works" etc. by which one could then say makes them therefor of Jesus and not of Satan.
Indeed for always we should remember this situation:
"And John answered and said, Master, we saw one casting out devils in thy name; and we forbad him, because he followeth not with us.
And Jesus said unto him, Forbid him not: for he that is not against us is for us." Luke 9:49-50
In practice I understand what you say to be true in an of itself, but not in answer to my question. However, without taking a stance in being able to call the specific institution what it is, we will not be able to reason why seeking one that is closer to our Lord in practice (works) and scripturally is advisable/warranted. Nor warn others.
What the institution holds out as its doctrines and beliefs and practices as a whole or a their primary essentials may make it impossible to much or even anything good about them, but may there not be there some individual hearts among them that please God?
Remember Rechab of Jericho,and Naaman of the Syrians, and the centurion [Roman?] of whom Jesus said,
"When Jesus heard it, he marvelled, and said to them that followed, Verily I say unto you, I have not found so great faith, no, not in Israel." Matt 8:10
I share this verse with you of which you are probably familiar with. Yet, the church of man by this very verse, keep many in chains, for their churches are eager to maintain what they have, which has nothing to do with the Kingdom of God. "Eager to maintain the unity of the Spirit in the bond of peace. There is one body and one Spirit—just as you were called to the one hope that belongs to your call— one Lord, one faith, one baptism, one God and Father of all, who is over all and through all and in all."
We should be very slow to speak adversely against anyone or anything, until and if we have heard from God on it. We are as the "good" Samaritan to love our neighbor, whoever he happens to be.
To our brothers and sisters who are or are to be part of the Body of Christ, we must be very closely connected, closer than to our neighbors who are not or not to be a part. But until we know, if we ever do, who is and who is not, what then? Remember God's Word to Samuel:
"And the LORD said unto Samuel, How long wilt thou mourn for Saul, seeing I have rejected him from reigning over Israel? fill thine horn with oil, and go, I will send thee to Jesse the Bethlehemite: for I have provided me a king among his sons." I Sam 16:1
When we hear as Samuel heard then we will be able to withdraw... but prior to that?
Sadly, too many are weak willed and accept what clearly is a barrier to knowing Him. Some churches do this more than others who also are barriers builders, but to a lesser extent.
So, my understanding from praying and reading scripture that the one Church is not a man made construct, but a Holy Spirit built, Body of Christ, with Jesus as the Head, eternal, yesterday, today, and tomorrow one.
Where I attend services regularly, there is much knowledge of scripture, much more than I have, but some of them to me are too strongly committed to saying "this is the Body of Christ" meaning our fellowship to the exclusion of others who do not regularly fellowship with us. This as I say and believe, is not good, but I've been around some, and I have seen no other place better. The key, of course, is to stay on the Lord's side, no matter what anyone else says or does. I am not the judge. But, if I keep submitting myself to the only One who really is in the end of the matter, will I go wrong?