We tend to think in man's terms. We forget that ONLY Heavenly Father KNOWS who has officially rejected the offer of the gospel in this life. I am a convert, and many of my family have not accepted the gospel yet in this life, and many have died without baptism, but I KNOW in my heart, and the spirit has born witness to me that they have not been penalized for it. Many have grown up with incorrect traditions and in an environment that has not been condusive to learning the gospel... Just as the Lamanites were not punished for the incorrect traditions of their fathers, and the Lord was merciful to them. The Lord will be merciful to those who did not have the OPPORTUNITY to accept it in this life.
That does not mean that they did not "hear something of the gospel in this life".. That means that they were not yet prepared... We can NEVER FULLY KNOW who is prepared and has REJECTED IT or has simply not been succored enough by the spirit to receive it. Your feelings are correct. God is NOT unjust. He WILL prepare a way for all of his children to understand, and THEN decide... They must KNOW what they are rejecting in order to be accountable. In other words, they must have a testimony and reject it to be denied entrance into the Celestial Kingdom of God. God is that MERCIFUL... Think of little children.. Do we punish them for what they do not know?? Wouldn't that be unfair.... God is true & just & merciful. He will prepare the way. We just need to be good missionaries, and love those around us and those who have gone before by doing the work for them in the temples. The Lord will do the rest in the spirit world for them... and YES they CAN enter into the Celestial Kingdom of GOD.