Can I use 1N4148 diodes instead of a 1N4001 diode for reducing the speed of a 5V 1.8W fan?
The 300 mA rating is in the Absolute Maximum Ratings table - you don't normally want to go near those ratings.

The 1N4148 is intended as a small signal diode.  If you look at the Electrical Characteristics table, you will see that most specifications are given with a 10 mA test current, so you should only use a 1N4148 with currents in that range.

As others have said, for diodes are in series, EACH diode carries the full current.