I like practical gifts of things I will buy anyway so for under the Christmas tree I want an egg candle so that I can see inside the shells of the unhatched eggs of each clutch next spring when the others from that clutch are 2 day old chicks. If the eggs are still alive I will leave them with mama if they are undeveloped or dead I will take them so she can focus if the chicks. I also need to be able to check duck eggs for double yolks so I don't allow those in a clutch for a broody duck, as I have 2 ducks that have had successful hatched prior to coming to my house.
I have some darker eggs. RIR, EE, etc
I'd recommend that you save your money. A candler is super easy to make. I simply use a little LED flashlight with fresh batteries, and occlude all light from the egg with my hand, except for a beam shining through the air cell. Any egg that is too dark for that to work, I use a table lamp! I take the shade off it, And use a shampoo bottle or a tube of cardboard, covered with tin foil to occlude all light. A pringles can would work well! Put a top on the tube, with an opening large enough to let enough light through so you can see the contents of the egg. Be careful, b/c the lamp method produces quite a lot of heat.
When ever candling, you have to be in a room that is pitch black!!!
You bring up some other questions though. How should I handle the eggs before setting and how many to give her? If they can collect 2 weeks worth of their own lay, I should be able to back track a couple days to give her a full nest.
Best to give her only as many eggs as she can easily cover well. If you don't need more chicks, only let her have 4 - 6 eggs! Save them up, and set them all under her on the same day. Until that time, you can let her satisfy her broodiness with a couple of golf balls, fake eggs, what ever. Mark all eggs, and place them under her at the same time. If you keep other hens away from her, they can't add to her clutch. This way, you'll be assured that her eggs are all the same age, and will hatch on the same day. Around day 14, you can steal them at night, candle them, and remove any duds.
There are articles in the learning center about managing broody hens, as well as candling eggs. My fav. LC article is "hatching eggs 101". A must read, whether you are incubating or going the broody method. It will teach you so very much about care of hatching eggs and their development.