How are Mono and Diglycerides metabolized without the Free Fatty Acids of Triglycerides?
Inside the intestinal lumen, di- and triacylglycerol are hydrolysed to free fatty acid and monoglyceride by lipase.

These diffuse through the plasma membrane of enterocyte where they will   used to resynthetize triacylglycerol, and ultimately the lipoprotein chylomicron for transport into the general circulation. 

Contrary to what your question seems to imply, the glycerol to which the free fatty acid are affixed is not necessarily coming from the food: glycerol is routinely produced by glycolysis. (This notably also happens in the adypocytes). There is no requirement that food contain a ratio three fatty acid to one glycerol to be fully absorbed - if the weight loss property of DAG are true, it may be through  a different mechanism.