Citric acid is a commercial grade heavy duty type of descaler mainly used on commercial vending machine. I wouldn't use it on domestic coffee machine, I use the liquid descaler solution you can by from any department stores that sell domestic coffee machines. They usually come in 250mls bottle and I mix about 150mls(you can use whole bottle if you think it needs it) in litre of water in the tank and you have to press the descaler button which will be lit up to indicate descale is due for at least 5 seconds. This will start the descale process and light won't go out until descale cycle is completed as per manual i.e. after the descale mix is used up it will stop because tank is low, you then rinse tank out, fill it with clean water and it will do the flush part of cycle to complete it and descale light should go out when tank is low again.

You certainly can. Just turn the valve handle located above steam wand anti-clockwise to steam position without pressing steam button and hot water will come out of steam wand.