> 1. How do I get the Polaris to play in Unison-mode
> over midi?...

What selections/controls do you manipulate to have it run in 'Unison'
mode from the keyboard?

> 2. As I mentioned the Polaris works well with the sequencer
> but when I operate it from my masterkeyboard it has
> some echoe-effect. The polaris plays one note as I
> press the key and repeat it when I release.

This really sounds like a midi echo/feedback loop. This could occur any
where in the MIDI network - but assuming that there is a direct
connection between the Master Keyboard and the Polaris, temporarily
remove the Polaris' MIDI out connection and see if it removes the
behaviour.