Are they omnipresent?

No.
If not, how can they hear prayers over all the world? 

I don't think they can hear "everyone at once" all over the world, no. 

When we are "in the Spirit" we are not limited as we are now in the flesh.  We will be able to do many things we can't do now.

How does Satan know everyone's Sins and weaknesses?  He isn't omnipresent yet he still knows them.