How to set up speakers in series and/or parallel for a minimum 4 ohm load on the amplifier

If you don´t mind to lose stereo sound, you can wire up the speakers in series resulting in one load of 5 ohms.

I think that a 2 ohm resistor is not a good solution because a common output voltage will be arround 10 volts and with current of 5 amp you´ll need a resistor of 50 Watts, which is pretty expensive. Maybe buying an cheap tweeter will be better and improve high frequency sound quality.

Looking in the datasheet I found that this IC specifically (TDA7924) have an short-circuit and overload protection. Using a load of 2.5 ohms may not be a problem if you don´t exagerated in the volume.

Some other thoughts, how did you measured the speaker impedance? If it was only using an DC multimeter the number you got (2.5 ohms) is only the resistance of the speaker. The impedance also needs to consider the inductor. If the DC resistance is 2.5 ohms probably the total impedance is alredy 4 ohms, so you don´t need any modifications on the circuit. The speakers whose impedance are not 4 or 8 ohms are rarelly manufactured.

Parallelling identical 8 Ohm speakers to get a 4 Ohm load (and the opposite: putting 4 Ohms speakers in series to get an 8 Ohm load) is normal practice.

"The amplifier's power rating is greater than the sum of the speakers" - That can get problematic: at full power the speakers might be damaged. You want the speakers (combined) rated power to be at least the maximum power produced by the amplifier.