Is it possible to script points' positions within a multixy?

Hello,

I would like to set the points’ positions at specific places when opening my session. Is it possible to do it?

yes, simply set the “default” property as follows:
[x1, y1, x2, y2, etc]

Great!

Will this send a matching OSC message to the widget’s target?

Will this send a matching OSC message to the widget’s target?

No it won't.