on your first point, I wonder: are the Greek authorities worried about the strain (I actually first typed "stain"!) on the ability of the police to effectively fight real crime while hundreds or thousands of its force are dedicated to the violent assault of peaceful protesters?

They oughta be. But that's not the protesters' problem.

Another problem is that police then know that they are relied on in that way in that situation, then they get to think that they have licence in other situations.

Any idiot can face a crisis - it's day to day living that wears you out.

   The protesters have to weigh every act and gesture for all the ramifications, meanwhile, the authorities feel, rightly so, free to resort to any and every tactic, no matter how low, in seeking to impose their view on a mass public which opposes them.

 (protesters): We can't hijack a fire truck and use it to clean and clear the CS powder out of the air during protests, 'cause if we do, meanwhile, we put some innocent homeowner's life or property at risk---not to mention that the authorities wouldn't hesitate to commit arson in just such an instance just to discredit such a manoeuver by the protesters since they don't give a damn for either  lives or property other than their own.  

  To which you'd probably retort, "This is reality.  Rather than moan about it, accept it and deal with it and, above all, get on with it."

  (I pity people who have to try and argue against you.)

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MORE THOUGHTS  DEFEATING CS (tear gases) USE:

    As long as the authorities can effectively use tear gas on peaceful protesters, they retain a decisive advantage.  

   To counter it, protesters are obliged to find and exploit its weak points.

 as we all know,

    The "gas" is applied by firing a projectile containing an irritant powder, dispersed by a compressed propellant gas.  To defeat it, the projectile must be quickly and effectively "smothered" somehow before it can disperse its contents into the open air.  To do this, one needs at a minimum

 1)  some protective gloves to ward off burns in handling the canister,

  2)  a place to isolate and contain the canister's gas and powder, preventing the spread--and which won't burn or melt in the process.

  So, for example, a protester needs gloves, a tin can (to hold the burning projectile) and a supply of medium size plastic garbage bags; the CS projectile goes into the tin can first and then immediately into the unfilled (uninflated) plastic bag, where the propellant is allowed to inflate the bag until its exhausted.  All of that has to be passed off to others ready to remove it from the area;  the can keeps the projectile from melting through the garbage bag--so it might need some minor (paper) insulation, too.  The point is to "catch and isolate" all or as many as possible of the CS canisters in the first seconds after they're fired.  They shouldn't be thrown back at police since that doesn't prevent the spread of the stuff.    This still leaves those damnable "spray gun" versions--seen in use in the photo in Talos's post above,

http://www.eurotrib.com/story/2011/6/29/101124/561#116

 of Mazwoer, but that's another matter.

  Other possibilities are using sand (dry or moist)to douse/bury canisters where they lie in the street---this is less effective since the gas can seep through the sand unless lots of it is available (a problem of weight) and the police can kick the canisters out the sand liberating them again to disperse the CS powder.

   So, to isolate the CS, protesters must come prepared to somehow both catch and isolate the projectiles and them get them safely away from the area before the cops can stop them doing that.

  I think a combination of adequate cans and plastic bags and a pair of gloves can help toward that.  The availability is ready and costs are minimal.

   Any other devices which can vacuum up or otherwise contain the gas and powder could be helpful if not confiscated by the police.  In any case, the techniques must be easy, low tech and low cost and adaptable to changing circumstances.  

"In such an environment it is not surprising that the ills of technology should seem curable only through the application of more technology..." John W Aldridge