Let's consider a few parallel sentences that use the aorist subjunctive + perfect tense, then the present subjunctive + the perfect tense.
1. Whoever drinks cyanide (aorist subjunctive) has received (perfect) a death sentence, and whoever is noticing (present subjunctive) the smell of almonds in a drink offered to him has been having (perfect) the chance to suspect cyanide and to save himself from death.
In this case, the first perfect tense verb indicates that the action of drinking cyanide immediately initiates an ongoing real result, from that present drinking, of receiving a death sentence. As soon as any person drinks cyanide they will have received a death sentence and that death sentence will continue as real for them from that time onwards. The second perfect tense verb indicates that for as long as a person is smelling almonds in their offered drink and has not yet drunk, they have the chance to avoid fatal poisoning.
2. Whoever practises (aorist subjunctive) satanism has sold (perfect) his soul to the devil, and whoever is recoiling (present subjunctive) from practising satanism, has been preserving his own control of his own soul from enslavement to satan.
In this case, the first perfect tense verb indicates that the action of practising satanism indicates that the person at some point in the past sold their soul to the devil, and is continuing in that state. And the second perfect tense verb indicates that for as long as a person is resisting the temptation to practise satanism, they have also been preserving their own control of their own soul.
Context may indicate whether we are looking at the ongoing result of a present action pr the ongoing result of a past action. But the structure itself seems to me to be ambiguous.
So, is Jesus saying A."Whose soever sins you forgive, they have been forgiven from the time you began forgiving them, and who soever sins you are retaining, they have been retained ever since you began retaining them ?
Or, is Jesus saying B. "Whose soever sins you forgive, they have been forgiven since before the time you forgive them (i.e. since the cross), and who soever sins you are retaining, they have been being retained ever since before the time you began retaining them (i.e. since the cross) ?
Or, is Jesus saying C. "Whose soever sins you forgive, they have been forgiven since before the time you forgive them (i.e. since the cross), and whose soever sins you are retaining, they have been being retained ever since before the time you began holding the offender accountable (i.e. since the offence happened) ?
Or something else?
I lean to interpretation A, and think Jesus is dealing with reconciliation between those offended and the offenders of the ones offended, not reconciliation between the offender and God.
When Stephen was stoned, he only granted forgiveness for the sin being executed on him, not for forgiveness for all his murderers' sins against others and God as wel (Acts 7:59-60l. When Paul and the Corinthians forgave the sexual sin practised by one of their own (2 Cor. 2) , it was a sin that had involved them too, since illicit sex by a Christian joins Christ's body (fellow believers) with the participants (1 Cor. 6:15-29).
I am of the view that Paul and the Corinthians were working to reconcile the offender to the church by holding hm accountable and relenting when he repented. They were not in some way forcing God to forgive the sin as far as it was committed also against God. Fir that the sinner needed to deal with God directly through Christ, as Peter told Simon the magician to do when Simon asked Peter the advocate on his behalf to God for Simon's forgiveness.
Act 8:21
Thou hast neither part nor lot in this matter: for thy heart is not right in the sight of God.
Act 8:22
Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee.
Act 8:23
For I perceive that thou art in the gall of bitterness, and
in the bond of iniquity.
Act 8:24
Then answered Simon, and said, Pray ye to the Lord for me, that none of these things which ye have spoken come upon me.