2 Thessalonians 3:17 isn't enough evidence to dispute Paul's authorship of the book of Hebrews, Paul being the apostle to the gentiles as clearly identified in his commission:
16 But rise, and stand upon thy feet: for I have appeared unto thee for this purpose, to make thee a minister and a witness both of these things which thou hast seen, and of those things in the which I will appear unto thee;
17 Delivering thee from the people, and from the Gentiles, unto whom now I send thee,
18 To open their eyes, and to turn them from darkness to light, and from the power of Satan unto God, that they may receive forgiveness of sins, and inheritance among them which are sanctified by faith that is in me.
The book of Hebrews in all clarity separated law and Grace and required one with proper divine commission and deep measure of revelation and deep spiritual understanding only given by the Holy Ghost, having a deep search of the word and by the Holy Ghost. The book required not just an instructor but one with an apostolic commission having the Mind of Christ.
We read in the scripture:
1 Corinthians 2:9-16
9 But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.
10 But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
11 For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.
12 Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God.
13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual.
14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
15 But he that is spiritual judgeth all things, yet he himself is judged of no man.
16 For who hath known the mind of the Lord, that he may instruct him? But we have the mind of Christ.
Paul had such a call for we read again:
1 Corinthians 4:15
15 For though ye have ten thousand instructors in Christ, yet have ye not many fathers: for in Christ Jesus I have begotten you through the gospel.
Paul, the masterbuilder was prepared by the Holy Ghost for the great task of laying the foundation of the Spiritual household of God
as it says in Romans:
1 Corinthians 3:9,10
9 For we are labourers together with God: ye are God's husbandry, ye are God's building.
10 According to the grace of God which is given unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. But let every man take heed how he buildeth thereupon.
This commission requires on called like Bezaleel in the old testament who was chosen and named by the Lord for the special work of building the tabernacle and the vessel's of the tabernacle. God , prepaired, anointed and placed Bezaleel for that very purpose as we read in the scripture:
2 See, I have called by name Bezaleel the son of Uri, the son of Hur, of the tribe of Judah:
3 And I have filled him with the spirit of God, in wisdom, and in understanding, and in knowledge, and in all manner of workmanship,
4 To devise cunning works, to work in gold, and in silver, and in brass,
5 And in cutting of stones, to set them, and in carving of timber, to work in all manner of workmanship.
6 And I, behold, I have given with him Aholiab, the son of Ahisamach, of the tribe of Dan: and in the hearts of all that are wise hearted I have put wisdom, that they may make all that I have commanded thee;
The strength of the superstructure ( building) , lays on the substructure (foundation). Through his apostle, Paul God was laying a sure foundation for the Spiritual house he was building:
We read again:
19 Now therefore ye are no more strangers and foreigners, but fellowcitizens with the saints, and of the household of God;
20 And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone;
21 In whom all the building fitly framed together groweth unto an holy temple in the Lord:
22 In whom ye also are builded together for an habitation of God through the Spirit.
The book of Hebrews was also addressed to the Jews to whom the temple worship. Paul's ministry was not accepted among the Jews as he was foreword by the Lord and as also evidenced from the book of acts:
17 And it came to pass, that, when I was come again to Jerusalem, even while I prayed in the temple, I was in a trance;
18 And saw him saying unto me, Make haste, and get thee quickly out of Jerusalem: for they will not receive thy testimony concerning me.
By the wisdom of the Holy Spirit Paul concealed writing his name in writing His epistle to the Hebrews, so as not to lay a stumbling block for them. As Paul acknowledges:
1 Corinthians 9:19-22
19 For though I be free from all men, yet have I made myself servant unto all, that I might gain the more.
20 And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law;
21 To them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law.
22 To the weak became I as weak, that I might gain the weak: I am made all things to all men, that I might by all means save some.
Yet again Hebrews 13:22- 24 strongly suggests Paul's authorship.
23 Know ye that our brother Timothy is set at liberty; with whom, if he come shortly, I will see you.
24 Salute all them that have the rule over you, and all the saints. They of Italy salute you.