Judas-necessary for salvation?

No, absolutely not.

So, Judas was not responsible for Our Lord's crucifixion. If this had been the case, then Judas (a man), would be partly responsible for salvation. But the scripture says otherwise —

“For by grace are ye saved through faith; and that not of yourselves: It Is The Gift Of God: Not of works , lest any man should boast.” [Ephesians 2:8-9; KJV]

The scripture emphasizes the fact that Our Lord laid down His life of His own volition. This should be clear when He said “No man taketh it from me, but I lay it down of myself…” [John 10:18; KJV]

If anything Judas’ death, shows that

1. One can be surrounded by truth and yet live a lie. Nearness can create a dangerous illusion of safety — “Yea, mine own familiar friend, in whom I trusted, which did eat of my bread, hath lifted up his heel against me.” [Psalm 41:9; KJV]
2. Our Lord desired to save him, though he was the “son of perdition” as He said in His high priestly prayer, for He chose Judas, who would betray Him as a friend — “For it was not an enemy that reproached me; then I could have borne it: neither was it he that hated me that did magnify himself against me; then I would have hid myself from him: But it was thou, a man mine equal, my guide, and mine acquaintance.” [Psalm 55:12–13; KJV]

Judas had physical proximity to Our Lord. However, proximity without devotion is perilous indeed.

i) Progressive — Judas had a front-row seat to miracles, heard the Sermon on the Mount firsthand and witnessed Lazarus walk out of the tomb. Yet, despite this incredible proximity to the Son of God, Judas remained spiritually blind and ultimately lost. The mere act of being in the vicinity without a change of heart does not lead to salvation. Salvation leads to Sanctification, which is progressive.

“For this is good and acceptable in the sight of God our Saviour; Who will have All men to be saved, and to come unto the knowledge of the truth.” [1 Timothy 2:3-4; KJV]

ii) Costly — Judas chose money over his Master. This led to the most heinous betrayal in human history.

“No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.” [Matthew 6:24; KJV]

So, no Judas Iscariot was not a “necessary agent of salvation” as you put it.