When you install multiroom, Astro installer will use back the same existing satellite dish as what you have right now at your house. Yes, thwy will provide second decoder and remote control.

If you thinking of taking the second decoder into another house, make sure the second house has satellite dish installed. Otherwise, you can purchase Astro satellite dish.

For Multiroom, no.
Of course they won't allow customers to do so, because Astro will lose a lot money if customers uses this method.

You can bring the second decoder to other location of course, but it will be in your own risk.
Basically, don't tell Astro that you're bringing the decoder to other location so that they won't be suspicious on you.