First, thank you for the points.
1) I do not have any material about MKS (meter, kilogram, second ?) system, but I think a quick search in the Google will solve it.
2) You are making some confusion about power definition. We do not say 400mW per second, it is wrong. Because POWER is the rate of energy (or work) consumed/generated/dissipated per time unit, so 1W = 1 joule of energy per second.
POWER = ENERGY (or WORK) divided per TIME
Does not exist such unit POWER divided per time.
So your IC has 400mW of dissipation, or it has a rate of 400mJ per second of heat dissipation, or in other words, it dissipates 400 mili joules per second (uses electrical current to make some work and generates heat).
Do not worry about this confusion, a lot of people does the same.