How scratch resistant is the lens?  

We use a material and coatings found in modern eye glasses (i.e. not glass, but a durable clear plastic with coatings.)  It will stand up to some abrasion, scratching etc, but care must be taken to protect it.  This sight comes with a neoprene cover to address these concerns.

Are there any coatings on the lens to minimize rain drop blur?

Yes.  But as we all have experienced, rain tends to leave dirt / debris on the lens.  Best to protect the lens from the elements if possible.  This sight comes with a neoprene cover to address these concerns.

Are there any coatings on the lens to minimize dust collection?

Yes.  This is a multi-coated lens, but there are limitations.  This sight comes with a neoprene cover to address these concerns.

How does one clean the lens to minimize scratching it?

The sight comes with a soft cleaning cloth.  If you lose this cloth, there are many suitable replacements.  Avoid paper towels, or other abrasive materials.  Water is safe to use for cleaning, stay away from harsh chemicals.

Is the lens replaceable if it becomes scratched up or damaged? 

The lens is not replaceable. The lens is wired in with the control / battery compartment.