I have spec C 150 type I/II portland cement. I usually do a 3-2-1 mix but need very small aggregate so didn't use the rock. I found info that said to do a 4/1 mix if using sand/small aggregate. At least that was the most consistent advice I found. I am using equal sand and ground granite (from my well drilling) with the cement in the 4/1 ratio to set around rocks laid on a concrete garbage can pad. I mixed it yesterday and today find this to be crumbly and moist beyond what is consistent with a 16 hour set. I can pull it apart with my fingernail.
Can I mix cement with sand and obtain a strong mix? Is something else necessary in the absence of rock or do I need more cement? What is mortar, then? That is basically what I need so can I make it or some semblance of it here with what I have?

Basically you are doing a mortar mix, sand and cement. I am suspicious of the ground granite mainly because I have never used it. In my work we used a simple 2:1 (2 sand, 1 cement) for mortar. It should set in a couple of hours depending on air temp. Mortar develops good strength around bricks or rocks but has little compressive strength, you need the bigger aggregate to develop compressive.