i have bolded the questions and ill answer them in order, first id like to say as some one who suffered birth defects due to the fallout from Chernobyl i have resurrected everything i can and learnt more than any one not in the nuclear industry on how that specific reactor worked and why the accident happened, i was also lucky to visit the site twice before it was enclosed in the new containment unit, once taking a tour of the control room for reactor 4.
1. yes but its not immediate, only 1 person died immediately ( Valery Khodemchuk, building collapsed on him) in the explosion, death by radiation is a slow and agonizing death over a number of days depending on dose one of the symptoms is a break down of the skin known as radiation burns.
2. the explosion wasn't a nuclear explosion, it was a an explosion from the pressure build up in the reactor that was release when it raised to the level it could blow off the reactor lid, like shooting a bullet from a gun, the radiation was then jettisoned into the atmosphere from the reactor and by the debris spread.
3. yes but no helicopters were knocked out directly by radiation, one at least went down due to the pilots suffering radiation sickness and hitting cables
radiation attack electronics by cutting tracks into the silicon inside chips and leaving charged particles inside the chip, these build up and can create bridges between circuit causing causing short circuit's but this takes time.
4. yes and likely more every day the reactor was open it was jettisoning radioactive particles into the atmosphere creating fallout that was landing as far away as Britain, further more the radioactive material was slowly melting down below the reactor towards the water tanks which if it reached it with water in would have triggered a 2nd (technically 3rd) explosion, teams on miners were brought in to tunnel under the reactor and drain the tank before it reached it.