Let's take the following proposition:
P1: If the devil were to repent, God would forgive him.
Now, on Christian theology, is P1 true or false? Moreover, if it is true, then is there a possibility for the devil to repent and return to heaven? Answers to this question will inevitably vary from one Christian theologian to another. However, let's assume that P1 is true. Does this make any more difference than P2?
P2: If there are square-circles, then the law of non-contradiction is false.
P2 appears to be true as a conditional statement, yet impossible to instantiate. This is because the consequent ("the law of non-contradiction is false") is necessarily false. However, that does not make P2 any less meaningful. We still understand P2 as providing information that corresponds to reality.
The question for the Christian theologian to ponder is whether or not P1 is like P2. Certainly God forgives those who repent, but is it possible for the devil to repent? This question is tricky because it depends on how one understands the manner in which redemption is made possible. Most orthodox Christian theologians accept St. Anselm's argument in Cur Deus Homo ("Why God Became Man").
When humanity sinned against God, only God could forgive us, yet only man could provide satisfaction for the sin. Hence, God became man in the person of Jesus Christ, who is fully God and fully man: one person with two distinct natures. His death "satisfied" humanity's sin against God, and allowed our repentance to lead us to redemption. Hence the saying: "Dying you destroyed our death; rising you restored our life."
If this account of the Incarnation is taken to be true, what it implies is that for any fallen angel, including Lucifer/the devil to be led to redemption, God would have to take on the nature of an angel and be both fully God and fully angelic. Whether this is possible can be left open to debate. However, I am personally inclined to think one's will is voluntarily fixed upon entering heaven or hell.
This would make any notion of repentance, much less redemption, for the devil impossible. If I am correct about this, then P1 is a true conditional statement that is impossible to instantiate. The difficulty is that the simultaneous truth and impossibility of instantiation of P1 is not nearly as obvious as that of P2.