(1) If you married someone not according to God's Will, would you be living in a constant state of sin as long as you are with that person? Would that lead to hell?
"God's will" is that you spend an eternity in heaven with him. That's it. God doesn't micro manage our lives. If he did, what would be the point of free will?
Your Mentor sounds to be a very wise man, according to his view on marrying "The One."
So if you except his Idea that marriage to certain people may be easier for us, than what life could be with others, then your question should be:
What should one do when they realize that they have married someone who will make life more difficult rather lessen the burden?
First I don't believe that there is a "Wrong person" you could marry unless they are not keeping there vows. Second anything after that will only serve to show you a percentage of how your relationship with God probably is from his perspective.
We often times aren't the spouse we claim to be during the engagement/salvation. So what does God do? does he divorce us because we are not good enough? or because we make life easier? No he sticks by his vow, and so long as we continue to come home to him he is faithful to us.. It is only when our self righteous behavior takes us to a place where we think we don't need God, or when we think to profit from God's mercy through not forgiving others as we have been forgiven, do we commit spiritual Adultery..
We do this when we put ourselves, or our needs in our hearts before God. The same example is ready to be lived in a marriage to someone in whom you may not be completely compatible with. Also know that one of you at least for a while will be the transgressor, and the other will be the forgiver.. Chances are if your not living in a situation (Post-honeymoon) Where your spouse is in constant need of forgiveness or work.. Then it is you, who is the burden.
So the question then should be, if your completely happy living with your beautiful wife, who cares for your every needs, should she divorce you because there maybe someone out there that may be a better martial fit for her?
The answer to your OP is that both of you being "Equally yoked" should strive to build a marriage based on what a solid Christian relationship with God looks like.. If both of you work toward this Goal with God as the head of your family then, no matter who you marry they will have blessed your journey or life in that they will have helped you understand the nature of God just a little bit more.. This understanding should be what marriage is all about!
(2) Would you be expected to divorce her/him in order to be even with God?
If you do you will have missed what Grace, understanding and forgiveness is all about.. And like the Servant who owed his Master a great debt and had it forgiven, but looked to collect a smaller from his fellow servant.. You too will be shown the grievous nature of that sin. Hopefully your heart is ready to except it, and you have time to correct it. Because the other time when you can be made aware of that sin is before our Master.