If you set up u/m as the base unit is each, then relate 50 each to a box, and 2500 to a case - you can buy and sell by the each, the box, or the case. Assuming those u/m definitions are for the same item.

Your wire item need the base unit set to inch, that is the lowest u/m you will use, then a spool is x=number of inches, you use/sell a qty of 4 to use/sell 4 inches

True QB only carries average cost to 5 decimal places, but only posts dollars and cents - nothing you can do to change that it is a limitation of QB.