Ok, another reason the pump could surge is apparently when the water level drops below the skimmer intake even though that line is closed off. I don't know how it could be leaking air into the pump if the valve is firmly in the off position but it apparently was doing just that. I turned the hose on and stuck it down the skimmer and voila! Pump regained and kept prime, no more surging.
Vacuum yesterday meant increased visibility this morning. I've just finished vacuuming again - this time the ENTIRE pool and the colour dramatically improved. I lost a lot of water since I'm still vacuuming directly to waste but progress is great right now. I also brushed down the sides of the pool which turned the water from grey blue to light green and then added another dose of pool clarifier. After things settle down to the bottom again it'll be time for another vacuum tonight and more water to top up. I got my first glimpse of the liner pattern at the bottom of the shallow end. It made me more happy than something like that should. I'm now going to scrub the top of the liner and try to clean the patio coping for good measure. If I'm lucky I may just be able to see what exactly is at the bottom of the deep end that I thought was the main drain. Both exciting and scary.