My concerns are probably unfounded, but if the amp will actually be putting out 2.5W into a 4 ohm speaker, I'd like to know. That could be both good and bad. I mean more power = more volume, but not if it'll end up blowing most small speakers.
I think you are making too much of it and over thinking the whole 'problem'. You know most amplifier designs deal with this by installing a volume control for the input signal such that the user can adjust the maximum power output of the amp. You haven't mentioned what kind of signals you are trying to amplify, music, sine-waves, square-waves, noise, etc? You can in your code have a means to scale the output signal range that will be supplied to the amp or wire a simple pot to the input of the amp and adjust it to 50% to start with and increase it only if needed. I suspect that the speaker you have selected will work fine with that amp.
Lefty