1) By your "past and present" observations using Rom. 1:18-23, in part you help substantiate my earlier general claim that "
Romans 1:18-32 is meant to have application for the church at Rome contemporary with Paul."
For this, thank you.
2) I'm not sure quite what you have in mind wrt "continuity" between--on the one hand-- the Rom. 1 indictment of humanity "past and present" from the vantage point of Paul when writing his Romans epistle and--on the other hand--1 Peter 3:18-20 (and textual environs), and my uncertainty is deepened by the 1 Peter 3 passage, which partly hinges on controversial and unfortunately in my view to date, enigmatic bits.
Be that as it may, Peter does seem to speak of Noah's ark as a type of salvation--a veiled sort of (typological?) prophecy of the salvation which Jesus would later accomplish on the cross (saved from the wrath of God).
And while I may not entirely understand what you mean by John 3:16 "as to mean all mankind past, present ,and future," I can at least say there seems sufficient grounds to say Peter viewed Noah (and family noted) to have been as saved as he anticipates his epistle readers to have been.
More broadly in Scripture, there seems sufficient grounds to claim that Abraham, for example, had a faith (cf. Rom. 4) like one who (say in Johannine terms) believes in the Incarnate Jesus, and that the eternal destinies of Old Testament saints (cf. the faith of some listed in Heb. 11) is the same as that of saints under the new covenant, of whom it may be said that to be absent from the body is to be present with the Lord--with the one Lord of all creation from the beginning until the present with projection into the future.
What this "B.C. + A.D. alike" salvation does not mean, however, is that Noah had the same revelation of Jesus--His incarnation, teaching, good works, cross work and resurrection, apostolic reflection--that we have. The Old Testament saints had "types and shadows" such as the sacrificial system, Noah's ark, prophecy of a coming King like David and Prophet like Moses, a bronze serpent lifted up, etc., etc. Surely God so loved, for example, Noah and Samuel and David and Isaiah that He gave His only Son on their behalf too, but the God and the promises they believed in, while real, were not as clearly revealed as after Jesus walked the earth. And while Jesus died for sins once for all, He died long after Noah or Isaiah were alive on earth. Noah need not have believed in Jesus entirely as we know Him to have been saved.
But also, such OT saints were sinners too, and knew pagans (non-Israelites) to have been sinners too. God destroyed the humanity He had made in the days of Noah because such people were sinners (Gen. 6:5-7).
As an aside, the Noah/Flood narrative often displays deliberate parallels to the earlier Adam/creation narrative(s)--which I note in case that points to an interest of yours wrt 1 Pet. 3.
3) Three remarks in closing. One is a reaffirmation of my uncertainty that I here "scratch your itch" or go some length in addressing your concerns. I may by guesses have drifted from your intent.
Second, the 1 Pet. 3 passage is concerned with using the-Jesus-who-suffered-unjustly as a model for the Christian (when so called) to suffer for righteousness sake in all good conscience, whereas Rom. 1:18ff again is concerned with indicting (created) humanity, particularly the Gentiles. God's patience in the days "while the ark was being prepared" (1 Pet. 3:20) was concerned with His delay of judgment before the Flood; Christians who suffer for righteousness sake need a like patience in any delay before they are delivered from evil. Rom. 1:18-ch. 3 as I wrote above builds from universal indictment of humanity in sin to universal need for salvation in Jesus--justification "to everyone who believes, to the Jew first, and also to the Greek" (1:17, cf. 3:21ff).
The two passages (from Rom. 1 & 1 Pet. 3) are rather different in focus and direction even if there is underlying theological coherence.
Third, I fear our discussion--while I here attempt to address your question--drifts from the OP of this thread even if your question attempts to get at a particular logical (or not, as the case may be) deduction.