I am saying that remission, and forgiveness go hand in hand, which are tied up in the sacrifice of Jesus Christ. That is what I am referring to as interchangeable. Even so, the Gospel itself is a message of reconciliation that happens as a result of Christ's sacrifice that grants us forgiveness/remission of sin.
1 John 1:9 is not about keeping a tally, or list, of sins that you then bring before the Father (seeking forgiveness). Look at the Greek for confession. It literally means to "come in agreement with", and contextually you'll see that in the previous verse (and following verse) it says that a person claiming to be sinless is not of the truth, calling God a liar. So, then, what an individual is doing is admitting that they have sin, are a sinner. This isn't about keeping short accounts with God, confessing one's individual sins, and seeking God's forgiveness or else.
This is like the law of polarity, hot and cold, on and off. Light and darkness. Saved and unsaved. A person claiming to be sinless, never having sinned in their life, is clearly not saved. They must confess their sins, or rather sinful state ("come in agreement with") in order to be forgiven ("he is faithful and just to forgive us our sins, and cleanse us from all unrighteousness"). Evangelists are ministering on the streets and have a list of the ten commandments and question people whether they have broken any of the commandments, and every time the person confesses they have. They end up realizing they have sinned against a holy God, and are indeed a sinner in need of God's forgiveness. They come in agreement with God about their sin, and through the Gospel, see their need for Christ.
This is the way 1 John 1:9 should be understood, going to the Greek and looking at the context. Please notice in 1 John 1:2 there is another solution, in fact the only solution, to sin in the believer's life. Our Advocate with the Father, our High Priest. The propitiation for our sins. Jesus Christ, the righteous who shed His blood once for sin, for all time (Read Hebrews).
It doesn't even make sense to say sin confession grants forgiveness, when we have in 1 John 1 (and 2) three different methods of dealing with sin then. The first being walking in the light (1 John 1:7), the blood of Jesus Christ His Son will cleanse you of all sin, followed by this idea of sin confession for forgiveness (1 John 1:9; a misapplication of the verse) and then last but not least, Jesus as our propitiation for our sins and being our Advocate with the Father (1 John 2:1-2).
The first and last are united, in that walking in the light is not walking perfectly (sinlessness) but being in Christ (the polarity of darkness and light, see John 8:12). In fact, let me post the verse.
John 8:12 King James Version (KJV)
12 Then spake Jesus again unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life.
We are in the light that is Christ, who gives us eternal life, and being in Christ we are cleansed through His blood of all sin, and if we do sin, as 1 John 2 says, we have an Advocate with the Father (Jesus Christ, the righteous). The propitiation for our sins. Look even further in 1 John 2, and you'll see it stated even further about the reality of our sins being forgiven, saying "your sins are forgiven you for His name's sake" (1 John 2:12).
There is no sin in the believer's life that needs a seeking out of forgiveness from God because it would defeat the purpose of Christ being nailed to the cross, dying and resurrecting, and then His ministry as He ascended as our High Priest with His blood (being the sacrifice for sin).
Hebrews 9:23-28 King James Version (KJV)
23 It was therefore necessary that the patterns of things in the heavens should be purified with these; but the heavenly things themselves with better sacrifices than these
24 For Christ is not entered into the holy places made with hands, which are the figures of the true; but into heaven itself, now to appear in the presence of God for us:
25 Nor yet that he should offer himself often, as the high priest entereth into the holy place every year with blood of others;
26 For then must he often have suffered since the foundation of the world: but now once in the end of the world hath he appeared to put away sin by the sacrifice of himself.
27 And as it is appointed unto men once to die, but after this the judgment:
28 So Christ was once offered to bear the sins of many; and unto them that look for him shall he appear the second time without sin unto salvation.
By
Hebrews 10:10-12 King James Version (KJV)
10 By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
11 And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins:
12 But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God;
He would've suffered since the foundation of the world if our sins, all of them, didn't get remitted because He is the sacrifice for sin (Hebrews 9:26). Any sin that we may commit is taken care of through the blood of Christ. This is covenant based (Hebrews 10:15-18). We are in the new covenant. Where remission of these is, there is no more offering for sin.