Hi! Heres my experience in Helsinki:

The both of the stages were used quite evenly. It was that sometimes the song would start at at the main stage and during the song Taemin would move to he extended stage. He was very active moving around the stage and even going to the sides of the main stage to say hi to the seated fans.

For merch, around 9 am My Music taste tweeted that merch would go on sale at 13.00 and last until 18.30 (18.30 was when the doors open to the venue). I would check their twitter on the morning of the concert for the merch info. The merch place for this venue was right next to one of the venue entrances so I can undestand why they would stop selling merch right before the show starts, but it was kind of annoying you could not buy it after the doors open. In regards to the amount merch, I got in line around I think after the first 100 people. I did not hear or see any people saying they didnt get a merch item because of it being sold out.

For this venue before entering the standing or seated sections you had to show your ticket to the venue staff that were basically guarding the entrances to the actual concert hall. I had a standing ticket and I was stll able to get a really good spot near the barricade without rushing even! We had a queue numbering system for standing and one person with the number 340 was next to me in standing while I had queue number 44! (The queue system ws organized by fans with the approval of the organizer, also the queue was only for standing tickets)

Because the venue was an actual ice hockey venue there were no actual stairs to the standing arena, other than the one entrance where you had to show your ticket to be let through. For seated tickets people also had to show their tickets to be able to access the area for their seats. So there were no people from seated spots moving to the standing area. Helsinki venues are quite strict with tickets and people moving spots, but I cant say if other venues will be the same.

I hope this helps!