Eh allomancy is end positive, when you are invested with enough of preservation you can access power by using up metals right, that's what makes it positive. The fact that metals run out doesn't make this negative as its coming from outside. When you awaken an object yes you give it a load of breaths, but you can get them all back, what is used up is colour, that's not a part of you, that's just like metal getting used up, the strength of the colour even works like the purity of the alloys, less bright = less effect
This suggests it is end positive, as you don't lose power (the breaths are still yours alone to take out) it comes from an outside source.
The fact that returned and nightblood consume
Breath is a little awkward! However perhaps to power consciousness/sentience/life actually consumes a small part of endowments power, not entirely sure :/