Depending what happens after the light signal is received this may all be totally irrelevant, and/or the effects I'm talking about may be infinitesimal due to the tightness of the coherence requirement, but...

OK, I'm probably being dumb, but can you explain to me where on earth that 20 degree requirement comes from? 

Why necessarily harmlessly?

Well, if there were scattered light around, only a very small amount would reach the detector within whatever coherence window is set...  Plus it would be much lower level than the true signal...   

 

   Rough surfaces are perfectly capable of absorbing light.  It's the absorption characteristics of the ink that matter, not the colour light it reflects that matters too.  An appropriate ink would in general be totally be capable of absorbing the 780-790nm CD laser light.