Use orange juice instead of water for syrup to make Orangettes?
Letting the peel simmer in the syrup is to candy the peels, as they are too bitter to be eaten as is. In my opinion, adding orange juice to this syrup won't make that big of a difference. The peels in themselves has a much stronger orange flavor than the juice has. If it makes you happy, you can replace some of the water with juice. When not wanting to waste, I just eat the oranges.

As to the syrup, I understand you do not like to waste stuff, but it is just water and some sugar (with some extra flavor now), so I usually discard it. However, if you really wanted to, you could use it in several applications, but make sure you account for the correct ratio (orange sauce for a duck breast, maybe you could add it to a cake if the water is no problem, you could make turkish delight with it, etc.)