To give a concise answer to the question, can people encounter the one true God in nature apart from the Bible and trust in him for salvation, I would say generally no. However, I think we are situated in an interesting time in history that may make offer an appearance of difference.
First off however, I want to clarify my thoughts by answering the follow-up questions in reverse order. Is Jesus necessary for salvation? Absolutely, Peter makes this quite plain in his address to the high priest in Acts 4, "salvation is found in no one else" (verse 12). Jesus' death, resurrection, and ascension did the work of exposing, disarming, and defeating the evil powers as well as offering an atoning work on behalf of all sinners. His enthronement at God's right hand too points to the rightful king to which the Good News of a new kingdom points, a kingdom whose work is salvific. Were we to concede that natural revelation alone could reveal God in such a way as to offer salvation, Jesus would nevertheless be the necessary agent of that salvation.
As to the Spirit's role, where Christ is the agent of salvation, the grace we receive is by the Holy Spirit. The Spirit seems to take a major role in the calling of people to follow Jesus and knowing the one true God (cf. 1 Corinthians 2, 2 Corinthians 3, 2 Thessalonians 2). I think specifically of the Ethiopian eunuch who discovered the scroll of Isaiah yet he still needed the guidance and teaching of Philip to lead him to salvation. In the case of Philip, the Spirit is literally recorded as having instructed him to catch up to the chariot so he could explain the Scriptures. This parallels well with Paul's poetic phrase in Romans 10:13-15, arguing while anyone can call on the name of the Lord, unless they have heard of him, they cannot call on him.
This leads me to my final thought on this. It would seem, based on Erickson as well as anecdotal evidence, that the argument for encountering God in nature as the primary, or sole, revelation leading to salvation is primarily a Western one. In Paul's time, Western civilization was dominated by various forms of paganism. Yet today, while the West certainly has ancient Greece and Rome in its rear-view mirror, seems largely formed by a Christian imagination and ethic. This is largely the focus of Tom Holland's recent book Dominion (which I am only about 2 hours into but he lays out his thesis in the preface) that Christianity formed a cultural imagination that has shaped much of our Western understanding of morality and ethics and has even embedded in many a latent imagination of the Scriptures. This is certainly not to argue that people are saved by the culture (and certainly not that culture embodies these ideals well) or that there is not still a need for Christ's saving work personally intersecting and interrupting your life. But it would seem that the question of whether or not we can actually have natural revelation is muddled by the fact that many arguing for it have been so saturated in systems of government, academics, literature, and media whose conceptions of love, justice, mercy, hospitality, and generosity derive from Scripture.
Therefore I would argue that we are unlikely to have seen, or at least rarely witnessed, natural revelation in the West, perhaps not in the last 1500 years or so.