Homer wrote:
Whether Mounce is correct, or Spiros Zodhiates is correct, I am not qualified to judge. So at best it is a "tie" between "receive now" or "take in the future". And Jesus' statement that the Holy Spirit could not come unless He went away breaks the tie for me.

We don't have to hang upon a duel between grammarians. We may consider parallel examples of the aorist imperative in
John.
On one hand, we have the example at
John 4:7. When Jesus interacts with the Samaritan woman at the well, we find an aorist imperative: "
Give me a drink." This is presumably not a directive that implies to be fulfilled on some occasion several days or weeks later; this is a request for the general (though not absolutely coincident) present.
On another hand, we have the example at
John 5:8. When Jesus tells the man at the pool to "
Get up, take up your bed, and walk," once again we have an aorist imperative
(viz., “take up”). Presumably, a rather timely response was expected here - not something on the order of weeks or months hence.
And on another hand, we may consider the parallel at
John 2:7. When Jesus commands the servants at Cana to fill the waterpots with water, this is another aorist imperative. Presumably, this is also a directive that is intended to be fulfilled promptly, in the generally present circumstance.
So there is no grammatical reason why the imperative in
John 20:22 could not have been intended to have a prompt fulfillment.

Which brings us to the other portion, "
Jesus' statement that the Holy Spirit could not come unless He went away."
Let's look at a few verses here:
And he said this about the spirit that the ones having believed in him were to take, for a spirit was not yet [taken], because Jesus was not yet glorified. {John 7:39}
But I did not say these things to you from the beginning, because I was with you. But now I am going away to the one having sent me, and none of you asks me, ‘Where are you going away?’ But because I have said these things to you, sorrow has filled your heart. But I tell you the truth, it is to your advantage that I might go away: for if I might not go away, the paraclete will not come to you; but if I might go, I will send it to you. {John 16:4b-7}
Being, then, the evening in that day – the first day of the week – and the doors having been shut where the disciples were (due to fear of the Jews), Jesus came and stood in the middle, and saying to them, “Peace to you.” And having said this, he showed the hands and the side to them. Then the disciples were delighted, having seen the Lord. Then Jesus said to them again, “Peace to you. Just as the Father has delegated me, I also am sending you.” And having said this, he breathed into [them], also saying to them, “Take a holy spirit." {John 20:19-22}

Why does the text mention that he breathes into them?
(The verb here is used once in the Septuagint – in the passage where G-d breathes life into Adam.)

Has Jesus “
gone away” by the time he breathes into them?
John 14:28 = “
You heard that I said to you, ‘I am departing and I am coming to you.’ If you were loving me, you were delighted because I am going to the Father – because the Father is greater than me.”

Has Jesus been "
glorified" by the time he breathes into them?
John 17:1, 4-5 = “
Jesus said these things; and having lifted up his eyes to heaven, he said: ‘Father, the hour has come! Glorify your son, in order that the son might glorify you. … I glorified you upon the earth, completing the work that you gave me in order that I might do it. And now, glorify me, Father, with yourself – with the glory that I had with you before the world existed.”