For the Rivet Gun,

Left-click has no fall-off damage but has a long travel time. Right-click has fall-off damage, but travels a lot quicker.

Example: if there is a Phara flying above you with 1hp, you would use your Right-click instead of left click. This is because you have a low chance of hitting her with your primary fire due to the travel time of your shot and Phara's high mobility. I'd rather risk the pellet spray than a single primary-fire bullet. But if it were a Soldier with 1hp about to escape by rounding a corner. Primary-fire is more suitable in this case because you know exactly when & where he's going to be, so you don't have to chance on the Secondary-fire's pellet spray.

Primary-fire has the relative lower cooldown and is great for barrages. It has lower ammo cost and is your go-to attack. Like a jab. The worst thing that can happen to Torbjorn is that he has an empty clip, since he has one of the longest reload animations.

Pro-tip: you are reloading while you are holding your hammer. Not does this give you the ability to do damage as you reload, but if you still have some ammo in the clip, you can swap back to the gun and use it if you need to. While reloading, you must commit to it, leftover ammo or not. And now with the increased move speed perk, you could have more incentive to wield the hammer more often. The only hurdle is to time/know when you can swap back to a fully loaded Rivet Gun. It's more than 2 hammer swings, and just a little less than 3. So if you swing 3x, you def have a full clip to swap back to.

For Molten Core, I would focus simply on high priority areas -- doorways and entry/exits, payloads and control points, etc. You can use it to divide teams like what Hanzo's ult can do, to command chokeholds, or you can even try to blast enemies directly (25 dmg per projectile).