Yes, when you download, a copy is made by the sender and then streamed across the network.
No. Remember, a "copy" is defined in 17 USC 101 as a material object, containing the intangible work. Much as the 3D printing folks would love to be able to download material objects, they just won't fit in an Ethernet cord, much less a WiFi signal.
When you download, you, the downloader, are fixing the work into your computer's storage device. That device -- usually a hard drive or SSD -- is the copy, and the downloader is the one responsible. (Leaving it on a RAM disk wouldn't help; then the RAM is the copy since it's not simply passing through briefly in that scenario)
This was all gone into fairly well back in the Napster era.
It is a copyright violation to retain a bootleg copy of media but only a civil infraction whereas copying and distributing is a federal crime.
No. You need to take a look at 17 USC 501, which defines infringement as any violation of the copyright holder's exclusive rights. Copying is exactly as much an exclusive right under 17 USC 106 as distribution. But possession isn't actually one of those rights. A downloader is breaking the law by being the person responsible for downloading (no one forced them to do it, or took control of their computer and made it download stuff) which is the making of a copy. Mere possession of an unlawfully made copy is not infringing -- there is no exclusive right of possession. But as noted, since a copy is a tangible object, you'd have to find a disc or drive lying around on the ground -- you can't download things onto a medium yourself.
Criminal infringement is defined at 17 USC 506 -- it's basically just a matter of degree. If you infringe a lot in a brief period, now it's a crime. Or if it's done for commercial gain, or if it's a work not yet released to the public.
Further it isn't just harder to 'go after' individuals because there are so many. It is harder because with a civil offense a party is claiming financial harm and asking for recovery... they have to prove the financial harm and are generally limited to the damages and in many cases this wouldn't include recovery of legal fees.
Nope. For the sorts of works people usually pirate, they've been registered timely relative to the infringement, and so now remedies available under 17 USC 504 include statutory damages, which do not require proof and which are $750 to $30,000 per work infringed, and can jump up to as high as $150,000 per work if the infringement was willful, which it almost always is. An 'innocent' infringement drops the minimum to $200 per work but it's rare that an infringer, even a downloader, can prove that they qualify for that.
Clever plaintiffs will only ask for the $750 minimum per work -- but for thousands and thousands of works -- to remove the determination of the amount of damages from jury consideration, but still squash the defendant. The Tatelbaum case back around 2010 or so was a good example as I recall. Also he had a crummy lawyer, but that's no excuse.
I feel like you haven't actually read the Copyright Act much less relevant case law. If you're going to make these claims about what is or isn't illegal and what the penalties are, you really should become familiar with it first. Don't make crap up, especially if you're understating the law which might result in people breaking it. Which is super easy for copyright which is a strict liability statute, and so your mens rea doesn't usually matter.