Per the supplier, there is no known shelf life.
Yes, it is the same product.
Any solvent (expect water) will suffice. The vendor uses alcohol for clean up (Ethanol/Ispropyl). Acetone, toluene, mineral spirits, etc. will work as well.
The light weight is a flexible compound for use as a joint sealing compound, caulking compound, corrosive preventative, lubricant, threaded joint dressing, insulating material, or filler with packing.
The medium weight seals with a gasket or as a gasket. Seals pumps, tanks, pressed plates, covers, pans, etc. for pipe threads and imperfect or uneven surfaces.