The Bible makes it clear in many places that whoever believes in Jesus will have their sins forgiven and will be saved. But is this forgiveness conditional or unconditional? And if it's conditional, then what are the conditions? Well, Hebrews 10:26 says that when you learn something is sin, but deliberately continue in it anyways, then you have no atonement for your sins. In fact, if you keep reading past that verse, you'll see just how dangerous it is to rebel against God's commandments once you learn what they are. The very next verse literally says that the enemies of God will be BURNED! UP! Jesus himself said, "Stop sinning or something worse may happen to you." [John 5:14; NIV] And we know from a previous examination of Scripture what that "something worse" is.
So one of the conditions is that we cannot continuously, deliberately do something that we know is sin. If we learn that what we are doing is a sin, we must stop if we want to be forgiven. Is that the only condition to being forgiven? No. Jesus made it very clear in several places (eg: the Lord's Prayer) that if we want God to forgive us of our sins, then we must forgive those who have sinned against us. In other words, if you hold a grudge against someone who wronged you, even if they have repented, you will be counted among the wicked and the fate of the wicked will fall onto you.
By the way, what does it mean to forgive? Psalm 32:1-2 gives a good definition. It means to not count or hold something against someone. When God forgives us of our sins, he no longer counts those sins against us. If someone offends you or harms you, and you forgive them, then you no longer count that offense or harm that they have caused you against them.
Are there any more conditions for our sins being forgiven other that what we discussed, and the need to continuously believe in Jesus? There does appear to be one more condition: "One can say something against the Son of Man and be forgiven; but whoever keeps on speaking against the [Holy Spirit] will never be forgiven, neither in the [present age] nor in the [coming age]." [Matthew 12:32; CJB]
Note: Because of the translation I quoted from in the above passage, I translated the Hebrew phrases in the translation to English. The word I translated as "age" can also be translated as "world" or "eon".
So, blaspheme against the Holy Spirit means complete disqualification from having your sins forgiven. Now, nearly all translations that I know of does not have the word "keeps", implying that one mistake in this regard condemns you with no hope of redemption. However, it appears that the Greek word translated as "against" (kata) has something to with a continuous action. So the CJB is probably correct to include the word "keeps". So one mistake won't do it. But the continuous action of blaspheming the Holy Spirit will indeed disqualify you from eternal life.
So, there are three conditions for being forgiven, not counting the obvious condition of believing in Jesus. These conditions are: (1) When you learn of your sin, stop deliberately doing it. (2) Forgive those who sin against you. And (3) do not blaspheme the Holy Spirit. What about all of those "what if" scenarios concerning sin? Well, if you're not deliberately sinning, then you show that you do not want to sin. The fact that you do not want to sin proves that your faith is the type that God wants from you: A faith that drives you to naturally obey his commands. Jesus himself advocates for us [1 John 2:1-2].
Those who learn of their sins, and initially deliberately continue in it, but later stop doing it and repent of it, they will also have complete forgiveness of their sins. But if anyone uses that as an excuse to start deliberately doing what they know is sinful, because they can later "stop and repent", is that repentance even genuine, since they planned to sin from the beginning? Such people put themselves into great danger! Nobody who genuinely believes in Jesus wants to sin. Nobody! They all want to live in full obedience to his teachings!
One more thing: If for any reason, there is a command that you cannot keep, then failure to keep that command is not a sin. Using this as an excuse to break a command that you can keep however, is a sin.