Very interesting fact: I have often wondered how it is possible that the kill feed goes to one of them, when I am sure that I've shot the enemy. 
I notice this regularly, and have always assumed it's a hosting issue. It's always particularly strange when I manage to get my Krogans into Rage-mode when they haven't actually been awarded the melee kill.
As for your wider concerns about your score being low when playing with those others, welcome to the club. That said, I don't give a crap, and if I happen to fluke a 5 headshot medal during extraction, woo-hoo!!!
At this stage, I know I'm playing the game 'right', as a co-op teammate, and that's all that matters.
My recent highlight came after being 'rejected' by TTT - I took my TSent / Phaeston into a random pug and got Condor/ Reapers. The time I'd spent idling, waiting for a call to arms, checking my calibrations, all paid off in the end, and Palaven was defended with some style and panache. I suspect I may even have managed more than 10 Headshots. 