So I assume the following:
...after pressing the ALT.A knob the ALT mode gets armed (is there any sign that shows that?) Yes, a little light next to the ALT.A Knob - the small circle between NAV & ALT.A in your photo until you are 300ft within selected alt. Then ALT mode engages and the ALT button illuminates "ON". Is that correct so far? Yes
I furthermore assume that you can control the v/s in ALT.A mode with the hat switch on the collective No, the hat switch on the cyclic - it's a 3 axis autopilot so everything is done via pitch & Roll - the Hat Switch on the collective steers the landing light.
and that ALT.A mode gets disengaged and current altitude will be hold if you press the ALT button BEFORE the selected altitude is reached?
Yes, but not very tidy if you have a high rate of climb on as it is done with a pitch change not collective.
Just in case these assumptions should be correct, does the V/S button illuminate "ON" while ALT.A mode is active? No.