How to account for phase and shift when composing a new signal based upon 2 or more recordings?
In EE, waveform mixing is used for wave modulation. It is implemented via mixer circuits whose operation principle can be loosely described as multiplication of currents. It does not apply to your scenario of SPL data acquisition and processing: you have no waves that modulate one another.
Your sound sources being independent and also removed at distances larger than a sound wavelength from each other (1.5 m at 1kHz in water), you can safely ignore phase effects and, specifically, acoustic interference. You can model the composite signal SPL as a linear combination of SPL's of your prerecorded signals.