Can A True Christian Be "Unsurrendered" To Christ's Lordship?

Unless you're willing to say that true Christians never knowingly sin, then you have to answer the title-question "Yes." Every time a Christian knowingly sins, that Christian has broken out from under Christ's yoke. If surrender to Christ's Lordship was an automatic, constant, and unchangeable result of being born-again, then Paul had no good reason to write us Romans 12:1, or Peter to write us 1 Peter 3:15a. 

But how do you tell the difference between a person who was never a genuine believer, and a backslidden Christian?, some might ask. One is that there will be something fundamentally wrong with the false believer's ideas about Jesus Christ and the Gospel, a deep, core flaw that had always been there. 

Example:  Jesus Himself had "disciples" who left Him in reaction to His teaching about spiritually eating His flesh and spiritually drinking His blood (John 6).  When those followers were confronted by a blindingly clear statement from Christ about their need to have Jesus Himself enter them by faith, the inadequate notions those "disciples" had about Christ was corrected. That enlightenment could have saved them! But instead most of His disciples left Him.

In other words, they walked away from Jesus Christ over Him.  They didn't leave Him over His teachings about adultery, or tithing, or whether Christians ought to drink alcohol. They abandoned Him over what He said about Himself, that He is the very Life of God. That was the difference between them and Peter, who backslid for a moment out of worldly fear.