Soteriology of the Old Testament is also an interesting thing. There were persons who could say, as in the Psalm, that afterward God would receive them unto glory.
This seems to imply some kind of private Holy Spirit ministry, which coexists uneasily with the scripture about a preacher being needed, but again in the extreme instance the Lord can personally be that preacher. He engages men too in that role not because He is logically forced to, but because in love He desires their involvement. We have left legalism and stepped into love, which in the end achieves everything that the law stated was necessary.
Yes, if He has someone elected for salvation, He will not be deterred if the person(s) He has delegated to preach to them proves unreliable.
About twenty-five years ago a suicidal man walked around Lake Elsinore in Southern California, which has about an eight-mile circumference. He told God that if He is real and didn't send someone to Him with encouragement by the time he got all the way around, he would kill himself.
A Christian man was driving on the nearby 15 Freeway when he felt a strong prompting to pull off, so strong he couldn't shrug it off. Uncomprehendingly, he obeyed, only to receive further promptings that finally led him to park at the beach and then to approach a man walking along the shore. He felt led to greet the man and say God had sent him, although he said he had no idea why. The walker, who was just finishing his circuit, was astonished and incredulous and explained the situation. Our obedient intervenor led him to Christ on the spot.
Even if He must send Philip to the eunuch, God is never at a loss.
...some kind of private Holy Spirit ministry, which coexists uneasily with the scripture about a preacher being needed...
You're very clever to notice that tension, but I don't consider that an uneasy coexistence at all. God is always the ultimate preacher, but in our ordinary world, the one Paul had in mind, he uses us to preach, in order to build our faith. When He must do it Himself (as indeed He did with Paul!), that is only the exception that proves the usual rule. He uses the Gospel from Creation (cf Job), He uses our example, and He uses His Scriptures, in addition to our puny preaching efforts--and in all cases He uses His Holy Spirit. Whether it's in the womb, on the deathbed, or anywhere in between, the Lord always gets His man!