Ellen's argument is convincing: "For four thousand years the race had been decreasing in physical strength, in mental power, and in moral worth; and Christ took upon Him the infirmities of degenerate humanity. Only thus could He rescue man from the lowest depths of his degradation. {DA 117.1}

"For what the law could not do, in that it was weak through the flesh"--it could not justify man, because in his sinful nature he could not keep the law--"God sending His own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh: that the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit." {PP 373.1}

Likeness of Sinful Flesh. There is a common idea that this means that Christ simulated sinful flesh; that he did not take upon himself actual sinful flesh, but only what appeared to be such. But the Scriptures do not teach such a thing. "In all things it behoved him to be made like unto his brethren, that he might be a merciful and faithful High Priest in things pertaining to God, to make reconciliation for the sins of the people." Heb. 2:17. He was "born of a woman, born under the law," that he might redeem them that were under the law. Gal. 4:4, 5.

He took the same flesh that all have who are born of woman. A parallel text to Romans 8:3, 4 is found in 2 Corinthians 5:21. The former says that Christ was sent in the likeness of sinful flesh, "that the righteousness of the law might be fulfilled in us." The latter says that God "made him to be sin for us," although he knew no sin, "that we might be made the righteousness of God in him."

"Compassed with Infirmity." All the comfort that we can get from Christ lies in the knowledge that he was made in all things as we are. Otherwise we should hesitate to tell him of our weaknesses and failures. The priest who makes sacrifices for sins must be one "who can have compassion on the ignorant, and on them that are out of the way; for that he himself also is compassed with infirmity." Heb. 5:2.

This applies perfectly to Christ; "for we have not an High Priest which can not be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin." Heb. 4:15. This is why we may come boldly to the throne of grace for mercy. So perfectly has Christ identified himself with us, that he even now feels our sufferings.

The Flesh and the Spirit. "For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit, the things of the Spirit." Note that this depends on the preceding statement, "that the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit." The things of the Spirit are the commandments of God, because the law is spiritual. The flesh serves the law of sin (see the preceding chapter, and Galatians 5:19-21, where the works of the flesh are described). But Christ came in the same flesh, to show the power of the Spirit over the flesh. "They that are in the flesh can not please God. But ye are not in the flesh, but in the Spirit, if so be that the Spirit of Christ dwell in you."

Now no one will claim that the flesh of a man is any different after his conversion from what it was before. Least of all will the converted man himself say so; for he has continual evidence of its perversity. But if he is really converted, and the Spirit of Christ dwells in him, he is no more in the power of the flesh. Even so Christ came in the same sinful flesh, yet he was without sin, because he was always led by the Spirit. (Waggoner on Romans)

Although this is direct to MM, yes, of course, it's the same sinful nature. Otherwise Paul's point in Romans 8:3,4 doesn't make sense.

Waggoner comments:

Sin originated in self-seeking. Lucifer, the covering cherub, desired to be first in heaven. He sought to gain control of heavenly beings, to draw them away from their Creator, and to win their homage to himself. Therefore he misrepresented God,
attributing to Him the desire for self-exaltation. With his own evil characteristics he sought to invest the loving Creator. Thus he deceived angels. Thus he deceived men. He led them to doubt the word of God, and to distrust His goodness. Because God is a God of justice and terrible majesty, Satan caused them to look upon Him as severe and unforgiving. Thus he drew men to join him in rebellion against God, and the night of woe settled down upon the world. (DA 21, 22)

Yes, I think they're saying the same thing. I think "flesh" is less ambiguous than "nature." "Nature" can mean many different things, depending upon the context. "Flesh" is less ambiguous.

When EGW speaks of "sinful nature," AFAIK she always has in mind the fallen human nature which Christ assumed, as in the quote, "He took upon His own sinless nature our sinful nature." (Medical Missionary 181; from memory, both the quote and cite, so might not be exact, but is at least very close).

EGW wrote that because of sin man has a nature which will inevitably lead him to sin apart from divine help. It was this nature which Christ assumed. Had Christ not relied upon divine help, He also would have inevitably sinned.

Christ is the ladder that Jacob saw, the base resting on the earth, and the topmost round reaching to the gate of heaven, to the very threshold of glory. If that ladder had failed by a single step of reaching by a single step of reaching the earth, we should have been lost. But Christ reaches us where we are. He took our nature and overcame, that we through taking his nature might overcome. Made ‘in the likeness of sinful flesh,’ he lived a sinless life. Now by his divinity he lays hold upon the throne of heaven, while by his humanity he reaches us."(DA 311, 312)

This is fallen humanity with all its hereditary inclinations. He who was as spotless while on earth as when in heaven took our nature, that he might lift man to the exaltation of himself by his righteousness.(RH 10/02/00

However, the strongest argument she was pre-lapsarian is the historical setting in which she wrote and lived.