Can you explain what the crowns in heaven are, what they are given for? And does that not suggest works over grace?
I think E.P. Sanders summarized the works vs grace conundrum best when he said, “Salvation is by grace, but judgment is according to works. Works are the condition of remaining ‘in,’ but they do not earn salvation.”
When we look throughout the entire New Testament, works continually play a place of importance. Indeed, the sheep and the goats are separated by them, implying that not being obedient to Jesus and living out our faith with works could bring judgment on us.
Likewise, there are parables and statements told by Jesus and other New Testament writers that show reward in Heaven for going above and beyond. For example, in Revelation the martyrs get resurrected 1,000 years before everyone else. This is a confusing passage to nearly all scholars, but I think it shows a varied heavenly reward. I don’t know if it’s symbolic or literal, but it makes total sense to me that the Christians who had their life cut short in this age (because they followed Jesus to their death) will get an early start to life in the age to come as a reward.
Jesus also said to “store up treasures in Heaven.” He also told a parable about how he will continue to give more to those who use his gifts wisely, and when he returns, those who used their gifts will get even more while those who didn’t will lose it all.
Paul talked about our faith being like a house. When Jesus comes back, our spiritual houses will be subjected to fire. So long as we have the foundation of Jesus, we’ll be saved, but those who have more than the foundation will reap a reward.
It’s not that resurrection life is going to be good for some and bad for others. Salvation is still salvation and resurrection is still paradise. But those who give up more in this life are told they will reap a reward for it. Despite this truth, salvation is still by faith.
A good book on the topic of faith and works (one of my favorite books in general) is Salvation by Allegiance Alone by Matthew Bates. I’d suggest checking it out to go deeper. 
My second question is can salvation be lost or once saved always saved?
Yes, salvation can be lost. Jesus himself said that the elect could be led astray and the early church was used to false leaders coming through and leading church people away from the church.
God’s election of us does not guarantee our proper response to him, nor does it ensure our faithfulness. In the Old Testament, the entire nation of Israel was God’s elect people. Every Israelite was literally born into being chosen and elect to carry God’s name and will out in the world, and yet it seems like the vast majority of them chose to be unfaithful and worship false gods. Just as they can betray their chosenness, so can we.
Jamin Bradley 