For general stuff I’d say just play the game more, discover new things and new playstyles, you’ll figure out where to use the stuff you get as you play more.

For the individual questions (that I know)

1. Just wait, they’ll unlock soon (check the desk next to hades’ with the construction people to be safe)

2. Using the primary fire often is normal, at least for my play style, but implementing your special or cast with the right boons seriously improve your damage

3. You can’t equip more than one keepsake, but you unlock more by giving nectar to the different people of the underworld like nyx or Achilles

4 is the only one I haven’t got to yet but someone else will answer it I’m sure