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You can Double Roll by:

1) LA Canceling
2) Homing Emission (PA) Canceling (costs PP, but easy to perform many rolls in succession).

I think so, yes.

The thing with Gravity Bomb is that it has a smaller AoE than Stun Grenade, therefore sometimes it'll miss, and consequently Tactics Trap won't be triggered.

Of course this goes without saying,...

RaHu is the way to go.

Additional info you may or may not know after a long hiatus:

Make sure to abuse the living hell out of End Attract and Crafted Parallel Slider for Assault Rifle;...

Bind Bullet simply sucks, therefore don't get it (as well as Mirage Shot, Panic Shot, Jellen Shot).

In reference to the skill tree that you posted, Hunter Skill Tree is fine. However, Ranger...

Impact Slider: to close in on enemies, good & fast damage.

Parallel Slider: use it to evade or gain distance from enemies. Note, you can use it to travel not just backwards, but from side to side...

Other PAs to consider:

All Rifle PAs have their usage, but I think the main ones you should also include on your weapon palette aside from the ones you mentioned are: Impact Slider, Parallel...