Rob Self Levelling Compounds are miracle-drugs in some cases and work very well.
A single 50# bag will cover 50 square feet at 1/8" of thickness. In your case you should probably consider 1/4" of coverage because 1/8" of coverage would be difficult to deal with and spread accurately. You would also require the compatible primer first.
The roofing felt method of crack isolation has always been poo-poohed by the floor covering industry but it also has been known to work. This would depend primarily on what type of crack was being isolated. A small stress fracture would probably be okay but an expansion joint (crack) would be a different story and should be honored through to the surface.