Hey Man,
Im more of a computer tech than an rc guy so this is my kind of area,
Your powersupply should be fine, the reason they say higher voltages should be used whilst charging at higher rates is to keep the amps down. To have a higher wattage we can do one of two things, increase the voltage or increase the amps. More amps is more heat usually and as a result we can increase the voltage to improve efficiency. Your charger will adapt to the lower voltage and just up the amps slighty, it will run hotter but will NOT be a problem.

The voltage is fine, all power supplies will ripple the voltage slightly under load, this is called the voltage drop, or load drop. I would leave the charger at 12v because the powersupply will be at 12v most of the time or atleast very close. The poorer quality the Powersupply the more voltage drop there will be. Just leave it at 12v.

Your powersupply has 1 22 amp 12v rail. This means it can supply 22amps at 12v safely. You could set it at 22 amps on the charger however your idea with the safety margin will mean that the powersupply has a safe barrier. If it over currents it shuts off, no harm is done but you would interrupt your charging cycle.
Hope I Helped
James