Instead of a wallwart, I would go for something like a Meanwell power supply ( say their RS-25-24 ) set to 25v output ( at 1.1 amps ) the output is adjustable from about 22 to 27 volts I recall .
They are pretty cheap here in South Africa, and I see they have distributors in the US and UK.
You can wire up your 100 LEDs in 5 series strings of 20 in parallel ( if you say they all have resistors built in for 5 volts, you wont need other resistors )
Have a power mosfet to ground the cathodes for the pwm.
You can feed a 5v regulator from the 24v ( a 7805 will take 35v ) and it will run at about 1/2 watt to drive an arduino.
These PSUs are stable driving pwm LEDs , I PWM 5 amps to LED displays with no problem, and if you do make a short circuit, the trip just hiccups until you remove the short.