For decades I've used Duraglass in the sails followed by filler with zero failures. I prefer Marson Platinum over the mass marketed Rage. It applies and sands better. Another method is actually bridging that sail valley with a piece of metal, welding it, and grinding to virtually elimate any filler except a skimcoat.
There are different methods to close the gap on the quarters. They depend on whether the panels are GM or not. Some (most) repro quarters lack edge definition at the door jamb and their radii are too large. Welding overlapping spots on the edge allows you to make the edge crisp. Doing so will close the gap; a large radius gives the appearance of a gap greater than 3/16". Primer and paint thickness must also be taken into account when pre gapping. The other method involves slicing the quarter at the jamb and either closing or widening the gap. Even though it's near an edge and not prone to excessive warpage, care with heat when welding must be taken. Or another choice is adding 1/8" filler rod to the edge. Some add material to the door edge but a wide door hem looks bad. If your doors are original it's best to mod a repro quarter than the GM known good door.
Never use filler on an edge to establish gaps or for any kind of edge repair. Your moldings should be fitted using weld, not filler, to make them fit.
You may be referring to "All Metal". A clever name used as a marketing ploy to lead consumers to believe it's panacea and substitute for proper metalworking techniques. Duraglass is fine. Shortstrand, not longstrand.
3M Duramix control flow 08329 is good for the driprail. It's 2K and you need a dispensor gun. SEM and Lord Fusor offer similar products.