I think your hope lies with the youth. If you can get a serious outreach to the youth (high school and college) and get them on fire for the Lord and bringing in new Christians, then you stand a decent chance of taking back the denominations. The fact that some denominations are being taken over by apostates is a clear indication that there is not enough new growth at the grass roots.
In my experience young Christians are full of zeal and are for the most part even less tolerant of blatant sin than your average 40 year old. Young Christians are less likely to put up with apostacy than old died in the wool demoninationalists who, like sheepish democrats, are Methodists and Episcopalians for no other reason than that their parents were Methodists and Episcopalians.
You need to fill the pews with former atheists and former cultists and repentant sinners. Then and only then will these denominations have the votes necessary to throw out the apostates and take back the church for Christ.
So, my solution is outreach. Get out of the building and hit the streets. "Compel them to come in."
In principle, I agree with you that victory is available by outgrowing a non-reproducing minority.
Our polity,however, makes change very slow and the infidels can remain in power and practice nepotism for years and years more.
Many of the newly won to Christ, will not have that patience.
It would ALSO be good to have a legal strategy.