What soil to use to patch up hole for lawn/grass

[spiidey]

We've just removed a tree and the stump from our lawn.

We're intending sowing need seed where the stump/tree used to be.

However, obviously (!), where the tree/stump used to be, there is now a hole. We've partially filled this back in with the soil/grass that we lifted when digging the hole itself.

We're going to have to buy some more soil. What soil should we be using when doing this? Should we mix any sharp sand in?

Also, when we dug the hole we lifted some of the grass that was immediately around the tree/stump. Is it ok just to lump this back into the hole (but below ground level - e.g. 6 inches or a foot down), or will this cause problems later down the line (levelling/drainage?).

 

[spiidey]

We're also intending planting a couple more rhododendron's in this area and using smartedge around the rhododendron at about a radius of 40cm.

So on one side of the smartedge would be the soil for the rhododendron and the other side of the smartedge would be grass.

The rhododendron needs ericaceous (acidic) soil - are we ok to fill the entire circle within the 40cm smartedge radius with ericaceous soil (i.e. right to the smartedge), and then normal soil for the grass outwith this area? Or would the ericaceous soil being filled right to the edge of the smart edge kill the grass?