To give second input toyour origional question: candeling IS what you want to do to check the egg for development. It is super straight forward and a very reliable means of checking that is generally easy to pick up on. IF your egg has a dark shell, you might not be able to tell, but if its regular brown or white it will be easy.
Like previous said, just google it for methodology. I use my hand to gap a flashlight and the egg. Also, candle one of your eating eggs of the same color first so you know what an undeveloped egg looks like (spolier the yolk provides no shadow like you might assume).
As for gaining weight... im no scientist but I think one of Newtons Laws of matter discredits that as a possilitlity. The egg is a closed enviorment, not being supplied by outside nutrients such as a mammal baby is. Therefore matter will neither be gained nor lost during incubation, yeilding it impossible for the egg to gain or loose weight.
As stated by previous responder, candeling will not effect your hen rejecting the egg. It IS less disruptive to mama if you do this in the evening/night when they are calm for sleep and subdued by lack of daylight. It will also be easier for you to candle when its darker.
While it is best to have multiple eggs under her from the beginning, because things do happen and not 100% will make it to hatch on most occasions; it IS unwise to add new eggs at this point unless you want to remove yours. Once one baby hatches mama will need to take on an active role in raising bu showing the babies food and water, so mama will only comtinue to sit on unhatched eggs for a few days past the first hatchling. Also, hens may not sit for much more than 25-26 ish days anyways. They know how long it takes, and if you were to replace an incubating egg with fresh ones halfway through the incubation cycle, your hen MAY give up and abandon the eggs before they can make it to term. It is very possible for a hen to sit for a lot longer if shes really stuck in stubborn broody mode, but your chances are smaller if you add a week and a half to her three week time commitment.
IF your boss leaves a LARGE number of uncollected eggs in her nest boxes, she may trigger broodiness in one of her hens, its worth a try. I THINK thats what triggered mine but i cant say for sure. There were like 18 eggs in the box when she went broody.
Hope this helps