I bought a new rear hub, same exact size as old, original spokes were 281mm however spokes i bought by mistake are 284mm, can i still use them to rebuild? I have deep double walled rim with lots of extra space for the spokes to stick out, however spokes have thread smaller than the spoke diameter and spoke does not screw past spoke cap, so i don't know where the extra length will go?
Spoke caps i bought are not same length as original, bought two variants - first is longer and second is shorter than original spoke cap, does it matter?
The new hub means you'll need to do re-do your spoke calculation anyway, unless it's the same dimensions.
Nipples of different lengths have different amounts of thread. The only good way to address that fact is to take an ERD measurement based on the nipples you'll be using. Published ERDs are always simplifications for this reason.
If 281 was a good size spoke for a given build, good defined as having nearly complete thread engagement, 284 would be way too long, and into the realm where running out of threaded length would be probable.
If you have a shop local with a production type spoke machine like a Phil or Morizumi, making your 284s into 281s is a fast job. I've done this sort of thing for people with mis-ordered spokes before.