Data points about booking&boarding:

Driver said, when I asked, it doesnt matter if you book XTZ or TKU. He is instructed so. He keeps the schedule and doesnt wait if no-show at TKU and boarding at XTZ instead, or vice versa. This is great info. We can freely choose XTZ or TKU what suits best and then board any of those what suits best👍
Now he only looked at my mobile app, but said later they will get QR-code scanners.

EDIT: asked the driver now, he sends all boarded pax info to somewhere at AY, so I recommend not to skip the bus if someone thinks about that for some reason.
Also, on HEL end, bus will wait max 1 hour for connecting pax to Turku if late arrivals.