Yeah, Artillery Zeri can be just as great in other roles! I just find that playing around the walls in mid fits my playstyle best. The great thing about Prowler's claw is that even without its active it's still the best mythic for the playstyle because of its stats and passive lethality. It does however offer some great opportunities for outplays! You can use it to jump off the wall during an E wall ride straight to an enemy, either to avoid a messy situation at the end of the wall or to suprise the enemy in question with your abrupt movement. From there you will do 15% extra damage for 3 seconds which helps finish the job since your ult explosion is weaker than back in the AP Zeri days. That, or you can use it to jump over an opponent to escape over a wall behind them.

In short, don't worry if you're not comfortable with its active yet, it is still great purely as a stat item.

With your last question I'm guessing you mean whether First Strike is better saved for your W or Ult? If so, I'd say 99% of the time I use the ult withing 3 seconds of a W hit, so it amplifies both if the rune is up. But in all honesty I rarely think about my first strike rune after laning phase. I take the W shots I think I can land, and if I get a First Strike that's just a nice bonus. Again, I might have misunderstood your question. If so, please let me know.

It depends on when you can time your base returns best. Two dirks provide the most damage up front at 60ad and 20 lethality total, but if you have more than 2200 and can use up more of your money at once by buying one of the completed items plus parts of the second then go ahead! In the end the difference will be miniscule :)

Haha I'm happy to hear it! :D

If they are full HP, no. Even if you're very fed a single W is extremely unlikely to one shot a squishy without followup from R or an auto since the W nerfs. If they are at ~80% hp it is very much possible to one tap squishies if you're ahead, however. Keep in mind that a W+R can kill squishies instantly, and since you can make them go off at the same time, perhaps that counts as a yes?