Can I carb keg like this?

In order to carb at room temperature, you'd have to set the regulator to 30 psi or so, and hold it on the gas constantly for about 10-14 days.

If you want to carb it up with sugar, you can do that. Use 2.5 oz of sugar for 5 gallons in a 5 gallon keg, and it will be carbed up in two weeks or so at room temperature.

It will not carb up at 15 psi at room temperature, especially if it's not on the gas constantly.

 

Yes, purge of o2. I don't know your room temperature, but if it's 65 degrees, 30 psi would be right. If the room temperature is higher, you'll need more pressure. Consult a force carbonation chart to check the exact pressure needed for your desired volumes of co2 if force carbing.

If carbing with sugar, that amount (2.5 ounces) is my preferred amount of carbonation (about 2.5 volumes). If you want it more highly carbed, you will need a bit more sugar.