Drew, here is a link to answer your question about your lawn. The ABC extinguisher contains monoammonium phosphate and is pressurized with nitrogen. A MSDS, or I think they call it something else now, can be had on Amerex.com..
Not sure why I can’t copy and paste but the site is Vizagchemical.com. In summary, MAP or monoammonium phosphate is a key ingredient in agricultural fertilizers. My lawn is the greenest on the street.
As far as training with one, yes, discharging an extinguisher on a live fire is an excellent idea. Aim at the base of the fire and not the flame with a gentle sweeping motion.
Obtaining a charged extinguisher for training should be no problem. Every 12 years, this extinguisher requires a hydro-test which is usually more expensive on a smaller extinguisher like the 5 lb. and customers usually opt to purchase a new one leaving the service company, in our case, with literally hundreds of extinguishers each year to dispose of. They have to be completely emptied for a landfill to accept them as they are pressurized. We give folks a soft drink and a hug to take them. Most are just 12 years old and are in perfect condition.