Deeper understanding of Isaiah 50:10 “walks in darkness” phrasing?

First we should consider the opening of this section:

1 Thus says the Lord: Where is the bill of divorce with which I dismissed your mother?

The fact of there being no bill of divorce means that God is still committed to bring Israel back, regardless of her sins. The entire nation comprises "those who walk in darkness yet trust in the name of the Lord." Israel has fallen, and now its people must look to the Lord, not their own wisdom, for salvation. This theme continues in chapter 51:

Listen to me, you who pursue justice, who seek the Lord; Look to the rock from which you were hewn, to the quarry from which you were taken... the Lord shall comfort Zion, shall comfort all her ruins; Her wilderness he shall make like Eden, her wasteland like the garden of the Lord; Joy and gladness shall be found in her, thanksgiving and the sound of song. (51:1-3)

"Those who walk in darkness" are the same people as "those who pursue justice." In both cases, they need to look to God for guidance. This understanding of the phrase works equally well whether one interprets these chapters as messianic (Christ speaking to the church) or "merely" prophetic (God speaking to His people Israel through Isaiah during the Exile).

Isa 50:10, 11 - Who among you fears the LORD and obeys the voice of his servant? Let him who walks in darkness and has no light trust in the name of the LORD and rely on his God. Behold, all you who kindle a fire, who equip yourselves with burning torches! Walk by the light of your fire, and by the torches that you have kindled! This you have from my hand: you shall lie down in torment.

Note the deliberate contrast between walking by the light of the LORD and walking by the light of our own fire - the latter will lie down in torment.

Darkness is simply the lack of light (obviously). Both words are used in the literal sense, and the spiritual sense where, in the latter case (as in Isa 50:10, 11):

• "brightness"/"light" means to have spiritual enlightenment, eg, Prov 4:18, Isa 60:3, etc
• "darkness" means an absence of spiritual enlightenment, and lack of divine guidance, eg, Ps 82:5, etc

These all remind us of the oft-repeated verses:

• Ps 27:1 - The LORD is my light and my salvation—whom shall I fear? The LORD is the stronghold of my life—whom shall I dread?
• Isa 9:2 - The people walking in darkness have seen a great light; on those living in the land of the shadow of death, a light has dawned. (Quoted by Matt 4:15, 16 about Jesus.)

In the NT this is explained further as a metaphor of Jesus, especially, in the Gospel of John -

• John 1:4, 5 - In Him was life, and that life was the light of men. The light shines in the darkness, and the darkness has not overcome it.
• John 1:9 - The true Light who gives light to every man was coming into the world.
• John 8:12 - Once again, Jesus spoke to the people and said, “I am the light of the world. Whoever follows Me will never walk in the darkness, but will have the light of life.”
• 1 John 1:5 - And this is the message we have heard from Him and announce to you: God is light, and in Him there is no darkness at all.

Thus, Isa 50:10 uses a well-known Hebrew metaphor saying that people who walk in darkness do not have the spiritual light of the LORD and thus, walk in spiritual darkness.

The Pulpit commentary offers this:

Verses 10, 11. - AN ADDRESS OF JEHOVAH TO HIS CHURCH. Some suppose that the Church of Hezekiah's reign is addressed; others the exiles towards the close of the Captivity period. The first verse is an exhortation, encouraging those who fear God, but have insufficient light, to trust in him. The second threatens such as "kindle fire," or cause strife, with retribution. Verse 10. - That obeyeth the voice of his servant; that is, of "his servant" for the time being, whether Isaiah, or Jeremiah, or "the Servant" κατ ἐξοχήν That walketh in darkness. Not clearly seeing his way or knowing what his duty is, and so inclined to despond and doubt. Every such person is bidden to put aside his doubts, and trust wholly in the Name of the Lord, and stay upon his God. Hence light will shine in upon him, and his doubts will be resolved, and sufficient light will be granted him to direct his paths. Isaiah 50:10

That is, people who walk in darkness deliberately reject the light of the leading of God and come to spiritual grief.

Isaiah chapter 50 addresses to the entire nation of Israel, rather than focus on defined individual. If it had to choose among the three options given by the OP, number three is close but still not definite.

The chapter begins with the Lord's declaration, that He has not abandoned Israel nor is He incapable of saving them, it is Israel who has forsaken the Lord and relied on their own strength (Isa 50:11 "walk in the light of your fires"). Isaiah, in contrast, remains steadfast in his faith, even when his neighbors mistreated him (Isa 50:6). The Lord empowers him to stand strong against his accusers (Isa 50:7-9).

Isaiah 50:10 addresses to individuals who maintain faith of the Lord. Even during moments of despair, when they feel as though they are walking in darkness, remains steadfast and trust in their Lord. It is because the Lord has not forsaken them; He is capable of saving them. This echoes the initial argument presented earlier in Isaiah 50:1-3.

The individuals who walks in darkness, are those who have lost hope for their future. This represents a period when the nation of Israel was in despair because they had forsaken their Lord. However, a remnant of people like Isaiah will find the light by steadfastly their faith in the Lord. On the other hand, those who rely on their own fires will find themselves consumed like torches set ablaze, lying down in torment (Isaiah 50:11).

That God led the Israelites through the wilderness with a pillar of fire by night. Exodus 13.

Here they light their own torches to be the fire that leads them and so they are walking in the darkness regardless of the torches providing light.

That there was a pillar of fire above them at night which created an area of illuminated ground for which to stay inside and stay warm during the cold desert nights and not stumble around and trip over rocks.

That at day there was a cloud of smoke that cast shade below it for which to stay inside and be protected from the Sun during the hot desert days.

In Genesis 1, God separated the darkness from the light. To walk in darkness is to be separated from God. Hence the phrase, and he walked with God being the opposite of walking in darkness.