A diluted chlorine solution can be used to lightly scrub the surface but do a test patch first to ensure the solution does not damage your stone. Using a pressure cleaner will work also, be careful to start with a very low pressure.
Liquid nails or the equivalent adhesive can be used to fix to blue board or brick/block.
Care must be taken with installations above 2m.
When choosing a finish for a driveway it is best to lay your sandstone pavers on concrete or for very light traffic you could ensure that a sound road base is in place and a minimum of 50mm thick pavers are used. Any movement in the pavers will result in cracking.
Yes you can use sandstone but it needs to be sealed with a good quality sealant. There are many on the market; natural which does not change the look of the stone and gloss finishes.