"how does the ec135 handle alt hold below 60 knots?"
It doesn't - at 60 knots and reducing, the annunciator will flash 5 times to tell you it's about to drop out - if you do nothing with the collective it will do just that.
"When you push the ALT button, the AP holds the current altitude, doesn't it?"
Yes it does.
"How exactly does the ALT PRE (aka ALT.A) logic work? In other words: Which buttons/knobs do you push in which order, if you use the ALT.A mode?"
Rotate the ALT.A knob to the desired setting and then press it.