Fabricate doesn't cause the creature with the ability to enter the battlefield with +1/+1 counters already on it. For example, Weaponcraft Enthusiast will enter the battlefield as a 0/1 creature, then its fabricate ability goes on the stack. Players may take actions (such as casting instants) while the ability is waiting to resolve.

You choose whether or not to have your other attacking creatures get +X/+X as Cultivator of Blades's triggered ability resolves, not as you put it onto the stack.

The value of X is determined as Cultivator of Blades's last ability resolves. It won't change later, even if its power changes. If Cultivator of Blades is no longer on the battlefield, use its power as it last existed on the battlefield to determine the value of X.

If you can't put +1/+1 counters on the creature for any reason as fabricate resolves (for instance, if it's no longer on the battlefield), you just create Servo tokens.

You choose whether to put +1/+1 counters on the creature or create Servo tokens as the fabricate ability is resolving. No player may take actions between the time you choose and the time that counters are added or tokens are created.

If this creature's power is negative as its ability resolves, X is considered to be 0.