Last question I answer, since I’ve answered several of yours now without getting an answer to my original question and my time is limited.
Aorist subjunctive active = potential action (hence the “might” in the English translation).
Perfect indicative middle/passive = present state resulting from completed action (hence the English rendering as “are forgiven”/”are retained”).
When these verb forms are combined together in this context, together they refer to a potential action resulting in a completed state—i.e., if the apostles might do X (forgive/retain sins), Y is the resulting completed state (sins in a state of being forgiven/retained).
If the people did not repent their sins were retained.
We see another example of this in Acts 3:11-21.
11While he was clinging to Peter and John, all the people ran together to them at the so-called portico of Solomon, full of amazement.
12But when Peter saw this, he replied to the people, “Men of Israel, why are you amazed at this, or why do you gaze at us, as if by our own power or piety we had made him walk?
13“The God of Abraham, Isaac and Jacob, the God of our fathers, has glorified His servant Jesus, the one whom you delivered and disowned in the presence of Pilate, when he had decided to release Him.
14“But you disowned the Holy and Righteous One and asked for a murderer to be granted to you,
15but put to death the Prince of life, the one whom God raised from the dead, a fact to which we are witnesses.
16“And on the basis of faith in His name, it is the name of Jesus which has strengthened this man whom you see and know; and the faith which comes through Him has given him this perfect health in the presence of you all.
17“And now, brethren, I know that you acted in ignorance, just as your rulers did also.
18“But the things which God announced beforehand by the mouth of all the prophets, that His Christ would suffer, He has thus fulfilled.
19“Therefore repent and return, so that your sins may be wiped away, in order that times of refreshing may come from the presence of the Lord;
20and that He may send Jesus, the Christ appointed for you,
21whom heaven must receive until the period of restoration of all things about which God spoke by the mouth of His holy prophets from ancient time. Acts 3:11-21 NASB