ON HELL
There are several verses describing a "second death" of the soul. While I'm not comfortable ignoring the verses that speak of a more eternal existence, these verses show people dying once of their bodily form, and then a second death is termination of the soul. The first is temporarily painful, so the second might be as well. Death as end of life.
Revelation 21:8 Their part/destiny will be in the lake that burns with fire and brimstone, which is the second death.
Second Death - Baker's Evangelical Dictionary of Biblical Theology - Dictionaries - StudyLight.org
In Matthew 25:46, eternal correction/punishment/penalty could refer to a sentence that has lasting consequences, but is not necessarily eternal pain and suffering.
If this were the case, then the people who could handle life in heaven would have that invitation, and those who choose not to would cease to exist. We accept death of the body as inevitable after 80 years, so it is not so horrific to recognize there might be a death to the soul.
When Jesus tells the story of the deceased Lazarus, He speaks of a holding spot where both the just and unjust are resting underground, awaiting a later decision.
If a person decides that they don't mind a second death and will just live however they want, they will tend toward harming others -- even if they are just living carelessly.
THE UNREACHED
Your example of unreached millions is certainly worth considering. But many stories have been told of God reaching people before any missionaries arrived. Unreached people have had visions, visitations, found texts and media on their own.
I had to laugh when I heard that one Muslim country had a huge percentage of their population watching Christian broadcasting secretly, without telling family members.
When people have a visitation without the information, they might form their own conclusions and create their own names for what they have seen and experienced. Missionaries are able to explain more, and provide practical help, so their efforts are not completely senseless.
THE ESSENCE OF MISSION WORK
Many missionaries deliver needed medical service, food, dig wells, provide water purification systems, teach people to read in regional languages. They start orphanages to get kids out of scavenging garbage dumps, or women's shelters to train them in job skills so they don't need to resort to prostitution.
Missionary work is not all about changing people's minds -- they also try to improve quality of life. When Jesus told His disciples to go out to other towns, what did He ask of them?
Luke 10:9
And heal those in it who are sick, and say to them, 'The kingdom of God has come near to you.
..Whoever does not receive you, nor heed your words, as you go out of that house or that city, shake the dust off your feet.
Matthew 10:8
Heal the sick, raise the dead, cleanse the lepers, cast out demons. Freely you received, freely give.