1. It used to reduce itself in some early version, but doesnt do that anymore.
2.
https://www.warzone.com/wiki/Artifacts3. Most people seem to go for either 25% AP or +1 slot (a few choose even dig time reduction), but that pretty much depends on what artifacts you have and your play style. You definitely dont want to discard a legendary.
3-a. Yes, all wzib rewards reset and you can get all artifact rewards from there another time. The same holds for all hardened rewards: you can (even have to) gain them again.
Thats why super ascending is a quite nice way to get artifacts super fast (if you have supered already a few times):
* 2 epic, 2 rare, 1 uncommon, 1 common, 1 poor artifacts from wzib
* 1 epic artifact from hard ccc level
That sums up to 0.69 legendary artifacts without digging AT ALL.
Note: there is also a legendary artifact reward from wzib, but afaik only 3 ppl ever got it and it is usually considered not worth the effort as it requires 102400 BP