These serve as working definitions which I do agree with.
So speaking of us believers,
We do not deserve eternal life - God gives us eternal life - we receive eternal life by the grace of God, yes?
We deserve eternal condemnation - God does not give us eternal condemnation - we do not receive eternal death by the mercy of God, yes?
Yes
Aren't these both sides of the same coin? And if not, where lies the difference?
The difference is that God does not continue in mercy to those for whom their sins must be brought to account.
For those whose sins were brought to account by Jesus and are redeemed, God graciously sees them in Christ. Does Christ Jesus need to be shown mercy? No. Therefore God does not continue to display mercy to the elect forever. God sees the elect as being seated in the heavenlies in Christ Jesus. We are the body of Christ. This happens by grace, not by mercy.
Again please note, I don't distinguish particularly between grace and mercy. Nor do I believe man is saved by mercy alone to the exclusion of grace - in fact, that wouldn't be my language at all...it's simply to clarify it in your words. I simply happen to use the word mercy because that's what Scriptures makes use of in the passages I'm referencing. For instance, if Rom 9:15-16 had used the word grace, I would've gladly used it too. I'm simply quoting passages here - unlike you, I do not have different theological concepts based on grace and mercy respectively. To me, a person is saved by the grace and mercy of God, both necessary and both complementary.
All humans are spared their just judgment by the mercy of God. If God did not show you or me mercy, we could never have been shown grace. So, in that light, mercy is instrumental in our redemption. Yet, it isn't mercy that saved us. If it was, then people of every faith would have hope in the mercy of the Creator to save them from judgment. There would be no need to be made alive with Christ. God would simply hold off on His judgment for eternity. Justice would never be meted out, but mercy would keep you as long as God decided to hold off on His judgment.
But, grace is needed. God must choose to place his wrath toward our sin upon the substitutionary death of Christ Jesus. In so doing, his mercy comes to an end upon Christ and His grace is poured out upon us through Christ. It is by grace you have been saved and that not of yourselves...
I guess you see this season of mercy as the time period for an elect until he is reborn after which grace takes over? Is that a fair read of your beliefs on this?
Yes
If so, how long does the season last for the non-elect then?
Until God's day of judgment.
And how do you reconcile passages that talk about the everlasting sure mercies of God which obviously isn't for a season - Psa 103:17, Isa 55:3?
Where is the conflict?
Psalm 103:17 But the steadfast love of the Lord is from everlasting to everlasting on those who fear him, and his righteousness to children’s children.
Isaiah 55:3 Incline your ear, and come to me; hear, that your soul may live; and I will make with you an everlasting covenant, my steadfast, sure love for David.
And finally, what happens to a believer under grace - is he still dependent on the mercy of God or does he no longer need it? He still commits sin and deserves condemnation which he believes in Christ to take away and preserve from - isn't that still God's mercy (Psa 51:1) ?
Romans 6 is a wonderful passage about grace and how we should not sin so that grace would abound.
It is God's grace that saves us.
Given Tit 3:5, are you still hesitant to say - Certainly we are saved by mercy too? That's just direct word for word Scriptures right?
I provided the answer above.
Without God's mercy, we could never know His grace.
Those who perish in their sins know God's mercy for a season, but never receive God's grace. They die under the full wrath of God.
These questions are to show the inconsistency of reading mercy as unnecessary after a season and making it mutually exclusive to grace when they're both complementary - so please answer the questions to show how you reconcile them or acknowledge a consideration to change your position. We could then resume our discussion on regeneration.
Hopefully you recognize that I have responded to your questions. Hopefully you see how I understand mercy and grace as they relate to each other.
You may not agree ( that's fine), but at least you see how I differentiate between mercy and grace.