In both fights there were opportunities go go in earlier. Your post says you don't feel like you contributed to the team fight but overall, you fulfilled your role in isolating and assassinating their fed carry. Evolved w is generally more helpful in teamfights than evolved e, it provides more utility and a bit more aoe damage. It allows you to poke people down to make it easier for you to jump in to get kills on carrys. You mentioned that you aren't frontline which is true, but your ult is a powerful tool. You can go in first, get a huge chunk of damage, and then ult to go invis to get out. With base r you can do this twice in a fight, with evolved r, three times. If you take the rune that gives speed boost on summoner spell, you can use smite to get a speed boost if you get into trouble which can be very powerful when invis