There is a good reason to think that these are two events, not one. That is because the continuity is broken by "And when the days of the Pentecost were accomplished, they were all together in one place" (Acts 2:1) If the narrative still placed them in the Upper Room, there would ne no need to time and place reference.
But that is not really my argument. Of course, the Holy Ghost can appear at any time to anyone, as well as Christ, the Blessed Virgin or the saints. When it happens it is a miracle of God. The scripture, however, also records how things are ordinarily: the Holy Ghost was given to a select group, some of which became priests and bishops. They proceeded to baptize and teach others, who received the Holy Ghost through their mediation. This practice continues to this day.
There is a good reason to think that these are two events, not one. . .But that is not really my argument.I'm sorry if I misrepresented your point.
To be clear, it seems to me (and maybe me alone) that the events of Acts 1 took place over a period of time, and that the events of Acts 2 took place later, which accounts for the scene-setting you referred to.
And I, of course, agree with you that the Holy Spirit was given to a select group, which then baptized the 3000.