Incredibly unlikely that any pills sold as E have heroin in them, for all the reasons listed above.
IF heroin users had contact with any pills somewhere along the chain of distribution, this could easily cross-contaminate them. As mentioned, traces on fingers or table top where H was prepared. These traces WOULD be picked up on analysis. Drug tests performed on dollar bills in New York show most notes have traces of cocaine on them! These tests are ultrasensitive. This kind of contamination is very different to a pill being manufactured with heroin in it.
When heroin is taken orally, it is metabolised in the stomach and broken down into different byproducts before being excreted via the liver and kidneys. This is why it is not effective to swallow heroin. When heroin is injected, smoked or snorted, it is absorbed whole directly into the bloodstream and only later metabolised and excreted.
Finally, research shows approximately one in four people who try heroin become addicted. If someone who had never tried heroin were to use just a little bit on a daily basis, they would be physically dependent with a habit within days, and would suffer withdrawals when they stopped using. I have worked with many heroin users and every one said they thought it would never happen to them. Tolerance to heroin builds up incredibly quickly, so a regular user has to use more and more to get the same effects. Most people with a habit seldom get stoned and use just to stop the onset of withdrawals. So there is no answer to the question of how much heroin is an effective dose. Also, tolerance drops quickly - within days - once people stop using, which is why many people who have broken their habit and decide to have another hit, actually overdose and die. They whack up with what would have been a small amount when they were using daily, and they drop, because what used to be a small amount is now an overdose.
ON Gold CKs... these have been widely available this year. They also made many people very ill in Sydney, particularly around January. Hospital emergency departments have dealt with many of these cases and I have personal knowledge of many others. I DON'T know what was in them, but I would be very surprised if it was methadone. Firstly, the solid form of methadone is a tablet known as physeptone. (Methadone is actually a syrup.) There is a very a hot black market for physeptone tablets as is and they are really difficult to come by. I really cannot see why a manufacturer would waste time and money mixing these tablets up with other stuff and passing them off as E.
That's my two-cents' worth!