Truly you jest. Christ was the SECOND MAN ADAM. He was capable of sinning, otherwise you have no Redeemer who was tempted like we are, yet without sin. Jesus knew firsthand the drive of human nature toward sin, as His humanity was His battleground. The Second Man Adam accomplished what the First Man Adam failed to do. Since the first Adam was given choices to make, so was the second Adam, Christ. To say, or even imply, that Christ could not have sinned is to take Christ’s obedience to God and make it nothing. The obedience of Christ would be a sham, a lie, an untruth.

Look at Hebrews 4:15 again. The verse is very descriptive of our LORD. Jesus’ likeness to us as the Second Man Adam, means He was tempted and that temptation was total, but with one exception—namely sin. Likeness without sin resembles Romans 8:3—in the likeness of sinful flesh. The temptations of Jesus were more severe than yours or mine will ever be—but He never sinned. His first recorded temptation is that of the very Devil himself tempting Him in the wilderness (Luke 4). The One who was so tempted, so able to feel for those who are tempted, is able to help us who are now being tempted even to turn away from Him as He was tempted to turn from His Father and that His Father’s will (Heb 2:18). As Calvin said,

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From this also arises the comfort for our anguish and sorrow that the apostle holds out to us: that this Mediator has experienced our weaknesses the better to succor us in our miseries [Hebrews 4:15a]. They claim that it is unworthy to attribute to Christ something evil of itself. As if they were wiser than God’s Spirit, who harmonizes these two things! "Christ in every respect has been tempted as we are, yet without sinning." [Hebrews 4:15b.] There is no reason why Christ’s weakness should alarm us. For he was not compelled by violence or necessity, but was induced purely by his love for us and by his mercy to submit to it. But all that he voluntarily suffered for us does not in the least detract from his power. These detractors are, moreover, deceived in this one point: they do not recognize in Christ a weakness pure and free of all vice and stain because he held himself within the bounds of obedience. Our fallen nature, whose violent and turbulent emotions know no bounds, is without moderation. Hence, our opponents wrongly measure the Son of God by that standard. But since he was uncorrupted, a moderation that restrained excess flourished in all his emotions. Hence, he could be like us [cf. Hebrews 2:17] in sorrow, fear, and dread, yet in such a way as to differ from us by this characteristic
Now, that you should better understand the humanity of Jesus—for He IS a real person (which your definition of Him denies; 2 John 1:7), you must now look at His Divine being, for His Divine nature being very God of very God could not have sinned for God cannot be tempted with evil (Jam 1:13). Your definition of Christ fails to see the union of His two natures. You confuse the nature of Christ and the decree(s) of God. That Christ could not sin by decree does not negate that He could sin in His nature. Hodge helps us here:

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What those qualifications are the Scriptures clearly teach.

1. He must be a man. The Apostle assigns as the reason why Christ assumed our nature and not the nature of angels, that He came to redeem us. (Hebrews ii. 14–16). It was necessary that He should be made under the law which we had broken; that He should fulfil all righteousness; that He should suffer and die; that He should be able to sympathize in all the infirmities of his people, and that He should be united to them in a common nature. He who sanctifies (purifies from sin both as guilt and as pollution) and those who are sanctified are and must be of one nature. Therefore as the children were partakers of flesh and blood, He also took part of the same. (Hebrews ii. 11–14.)

2. The Mediator between God and man must be sinless. Under the law the victim offered on the altar must be without blemish. Christ, who was to offer Himself unto God as a sacrifice for the sins of the world, must be Himself free from sin. The High Priest, therefore, who becomes us, He whom our necessities demand, must be holy, harmless, undefiled, and separate from sinners. (Hebrews vii. 26.) He was, therefore, "without sin." (Hebrews iv. 15; 1 Peter ii. 22.) A sinful Saviour from sin is an impossibility. He could not have access to God. He could not be a sacrifice for sins; and He could not be the source of holiness and eternal life to his people. This sinlessness of our Lord, however, does not amount to absolute impeccability. It was not a non potest peccare. If He was a true man He must have been capable of sinning. That He did not sin under the greatest provocation; that when He was reviled He blessed; when He suffered He threatened not; that He was dumb, as a sheep before its shearers, is held up to us as an example. Temptation implies the possibility of sin. If from the constitution of his person it was impossible for Christ to sin, then his temptation was unreal and without effect, and He cannot sympathize with his people.

3. It was no less necessary that our Mediator should be a divine person. The blood of no mere creature could take away sin. It was only because our Lord was possessed of an eternal Spirit that the one offering of Himself has forever perfected them that believe. None but a divine person could destroy the power of Satan and deliver those who were led captive by him at his will. None but He who had life in Himself could be the source of life, spiritual and eternal, to his people. None but an almighty person could control all events to the final consummation of the plan of redemption, and could raise the dead; and infinite wisdom and knowledge are requisite in Him who is to be judge of all men, and the head over all to his Church. None but one in whom dwelt all the fulness of the Godhead could be the object as well as the source of the religious life of all the redeemed.
These qualifications for the office of mediator between God and man are all declared in the Scriptures to be essential; they all met in Christ; and they all were demanded by the nature of the work which He came to perform.

Hodge, Charles. Systematic Theology. Originally published 1872. Oak Harbor, WA: Logos Research Systems, Inc., 1997.

Reformed and Always Reforming,