Instead of Rom 5:18 teaching that Christ died for all without exception, it actually teaches limited atonement because the results following the actions of each head, One head being Adam, and the other Head being Jesus Christ, who we know is the Head of His Body the Church Eph 5:23
For the husband is the head of the wife, even as Christ is the head of the church: and he is the saviour of the body.
Col 1:18
And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
Now lets look at Rom 5:18 with that truth in mind
Therefore as by the offence of one[Adam] judgment came upon all men to condemnation; even so by the righteousness of one[Jesus Christ] the free gift came upon all men unto justification of life.
Now the all men Adam represented as head, by his offence as their head, it resulted in their judgment unto condemnation, so even so, the all men Christ represented as Head, the Church, by His righteousness, comprised of His Life of obedience to Gods Moral Law, and His Death as the Just penalty of any one violation of Gods Law, as a result the all men he represented to them came upon Justification of Life, its the free gift of eternal life Rom 6:23
23 For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
So the results following the actions of each head for those represented determines who the all men are so:
So then as through one trespass [Adam’s sin] there resulted condemnation for all men, even so through one act of righteousness there resulted justification of life to all men.
So the all men in Rom 5:18 b is limited to all men which as a result of Christ's Headship righteousness, it resulted in their Justification of Life !
And we know from other scripture that cannot apply to all men without exception ! What about those who shall never see life ? Jn 3:36
He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him.
Its impossible for any of them to be included in the all men who receive Justification of Life as a result of Christ's act of Righteousness !