Well, no, you’d need at least one of the weapons to be a scimitar, and to have used one of your weapon Masteries for scimitars. You can’t use your bonus action to attack with the Light property and use your bonus action to attack with Dual Wielder. However, you can use the Nick Mastery to attack with the Light property as part of the Attack action and use your bonus action to attack with Dual Wielder. The other Light weapon can be a shortsword though, which is probably optimal.
Yes you can. You can do this without nick if you don't change weapons between your action and your bonus action and the weapon is light. Treantmonk talks about this in his coverage of the feat.
The feat states it gives you an "Extra Attack" on your bonus action as in one more than you already have with your bonus action. Not having Nick means you have to take both of them on your bonus action and it means you must be using a light weapon for your bonus action. Having Nick lets you move a weapon with the light property to occur on your action (and let's you switch out that weapon to use a d8 weapon on the bonus).
assuming they use a rapier for dex or a longsword for str.
If I am optimizing I am using a Battleaxe for Topple and always picking strength
No extra mastery effect, since this strategy already uses both mastery slots on scimitar and shortsword.
A Fighter has 4 weapon masteries at level 5, so one more than he needs to do this.
Moreover, if you make your first action attack on your turn with your D8 weapon you can do it with 2 weapons (as noted earlier). You action would be d8 weapon, light weapon - bonus action (2nd light weapon, 2nd light weapon)
You don't need nick to do this.
This is all before accounting for Fighting Styles, of course. If we also consider Fighting Styles, the dual wielder gets an extra 8 DPR from the Two-Weapon Fighting style (+Mod to the two attacks that didn’t benefit from it)
Ok I finally got to this, yes I am obviously assuming the Two Weapon Fighting Feat.