Doubt about USB bus powered device
I'm afraid of the absolute maximum current that it can draw and what could happens if it draws this amount of current.


Absolute maximum ratings are "if this is exceeded then the device may be damaged", not specifying that they will happen. You do not need to worry about that number. You do need to make sure that your current draw is under 100 mA or 2.5 mA when applicable by making sure the actual current draw due to program execution, internal peripherals, and attached devices does not sum up to over the USB-specified limit.


  
  I understood that the host (the computer in this case) will decrease current …
  


No. The USB host cannot "decrease current" because it is supplying a specified voltage. What this means is that when your device is commanded to suspend, according to the USB protocol, it must reduce its current draw or be in violation of the spec.


  (4 &amp; 5). The rule (4) is sufficient to rule (5), so, why specification on rule (5) was necessary?


4 applies only when your device is plugged into a bus-powered hub. If it is plugged directly into the host then there is no bus-powered hub involved, and the 500 mA limit applies.