1. We know that apostles were baptized and accepted the Holy Spirit in Acts 2:1-4. What than happened in John 20:22?
Acts 2:1-4 is the fulfilment of Old Testament promises (like Isaiah 44:3, Joel 2:28), and the words of the Lord Jesus in Acts 1:4. In the following verse the Lord compares the baptism of John and His own baptism. John baptized in (this is what the Greek text says; not: with) water and the Lord baptized in (not: with) Holy Ghost (there is no definite article ‘the’ in the Greek, so it means the power or gifts of the Spirit). Through this baptism (in Acts 2) the 120 disciples were ‘bound’ together as one people of God and received Holy Ghost (without definite article) to pronounce the wonderful works of God (vs. 11). This is why Israel is sanctified, “to proclaim the praises of Him who called you out of darkness into His marvelous light” (see 1 Pet. 2:9).
In Acts 1:8 we read that the Holy Ghost would come upon them to strengthen them to do the things God wanted them to do.
In John 20:22 the Lord says: “Receive ye (the) Holy Ghost”. The definite article is omitted in the Greek. So, at that moment, they received power of the Spirit, so they could carry out the commandments the Lord had to give them (see vs. 23 and comp. with Acts 1:2). At that moment they were not yet baptized in Spirit like is written in Acts 2. They only received spiritual power to do their work as apostles of the Lord. Always remind that ‘The Twelve’ had (and will have – Matt. 19:28) a special service and responsibility in the Kingdom.
2. We know that in the New Life, after millennium kingdom, we shall have incorruptible bodies, and won't have diseases. What than the leaves for the healing of the nations are for, in Rev.22:2? Shall we have diseases in a new life?
First of all, these words have no direct relation with us, who by grace belong to the Body of Christ. We are citizens of heaven in our Lord Jesus Christ. They point to a distant future when old heavens and earth passed away. A new creation becomes visible (Rev. 21:1). In that new creation there will be seen the New Jerusalem as the centre of the earth.
Secondly, the word translated ‘healing’ comes from the Greek ‘therapeia’ in which we recognize the English word ‘therapy’. There will be no healing of diseases or something like that, for “the former things are passed away.” (Rev. 21:4). Those leaves will be for therapy to help the people grow up until they become perfect, which is the goal of God. For some (or maybe many) of us it may sound rather strange that people who entered the new creation are not yet perfect. But let’s think of our own situation. In Christ we are a new creature. That is a glorious fact! But read what the Bible teaches in Colossians 3, vs. 9-10. It says that we have put off the old man and have put on the new man “which is renewed in (the Greek says ‘eis’: into) knowledge (Gr. ‘epignosis’ = full knowledge) after the image of Him That created him.” Once we became a new creature we went into ‘therapy’ to learn about God and His plan, about the Lord Jesus and His wonderful work and so on. Now - as a new man in Christ - we are growing up to perfect wholeness (see Col. 1:28).
In future the people who enter the new earth to live there, will also get ‘therapy’. God will take care of them, heal spiritual wounds, teach them and make them growing up to that perfect wholeness also. In the end all and everybody in the new creation will be perfect and then God will have finished His mighty works of the ages. And in that everlasting glory all creature will rejoice in front of Him and praise His glorious Name!