Is it simply volts x amps?
Taken when? at maximum, when heated up, or on some standard standing
voltage, after start-up?
Would one maker's 100W bulb be the same light and power draw as some
other
maker's 100w ?

Not necessarily. One maker's lamp could draw more current to produce
the same output in lumens.

Uh... Ignoring power factor... Power is power. Law of conservation of
energy, right?