Even instrumentals have a melody it differs from a band with a singer in that an instrument plays the melody as opposed to the singer "playing" the melody with their voice.
Exactly. In my case there are usually at least 2 or 3 different melodies happening (between bass, 2 guitars, and synth) ay any given time, so we can usually practice without any one member (except the drummer) and still be able to work things out.
In fact, practicing without different members can be helpful as it can shift the focus to different parts at different times, and help someone realize that a part that works in one context (with another guitar, for example) might not fit with the drums, or the synth.