As an ADC, you want to be as far back as possible before a fight begins. Twitch is one of those rare assassin/ADC types, so he is a little different. He can be the type to stealth in and murder everyone. However, he is still and ADC and will pop if focused. So, for the scenario you are describing, I would stay in the back and wait to see Rengar or Jarvan jump in on someone before doing anything. Unfortunately, you kinda bait your team into eating all the damage for you while you stand back and kill everyone.
With your comp, I would wait for Blitz to land a hook and just kill whoever he pulls. Wait for Poppy to get a pin on someone and attack them while watching for Rengar, Jarvan, and Hecarim. If you see the ! mark, hit Q and run until he jumps on someone else. If Jarvan ults you, flash away if possible or accept your fate and go down fighting. Focus whoever is the closest to you while being safe. Obviously there are some places that just aren't safe period, but do the best you can. If you can take 2 of their people out or get them low before dying, you're at least helping.
ARAM is kinda it's own beast cause you can have teams of all ADCs or all tanks, so just do your best to itemize properly to counter what you can and do as much damage as you can. For example, I normally take Bandit in my mastery and will go a support item if I have some long range poke. Letting Poppy or Yasuo take the minions while you sit in the bush with Ancient coin or spellthief can actually rack up gold pretty fast. Buying a Wit's End might've been a good choice as well since the MR would help against their poke, it has Attack speed and on hit magic damage, which would ignore the armor they were building, and it's sorta a counter to thornmail cause you reduce the damage reflected back at you. I personally go Youmuu's or Bork on Twitch first with Runaan's second. Bork would've helped get through Hecarim and Jarvan later while Youmuu's would get through Fiddle, Rengar, and Xerath faster.
, while the enemy team had 
