Looking for confirmation on power supply project
The connection to ground is not required. Sometimes there is an actual connection to ground but like in this circuit the symbol is used only to state that node should be considered as ground.
A resistor in series with the light bulb would work. Pick one with a resistance that drops 7V (from 19V to 12V) at the current that your light bulb draws. If your bulb is a 1W, this resistor would be around 75 ohms and waste about 1/2W. I would recommend using an LED rather than a light bulb.
Yes, the voltmeter and ammeter are correctly placed. With the ammeter in series with the load and the voltmeter across the outputs of the power supply.
Rather than replacing C3, add your additional capacitor in parallel. As the capacitance of a capacitor is frequency dependent and smaller ones react faster to changes in voltage.