1. They *build* Star Roads. They have their own form of FTL that lets them basically cruise through realspace without any issues. Kind of like a hyper-Webway but without the walls being kicked in by Dark Eldar or whiny kids.
2. Less "thrown together" and more "hyper adaptive" - it essentially knows everything anything it consumes knew. Machines and lifeforms alike. And it shares all of that across all the Flood that exist instantly.
3. Nope, they don't show immediately. They have some sort of bullshit telepathic ability to make people do shit for them to weaken themselves for attack without ever showing themselves there.
4. Nope. They were there when the universe was created. They, I guess, would understand everything about how the 'Nids work, if you wanted to go the maximum bullshit route.
5. Yeah, kind of. I seem to recall it has some FTL bullshit going for it in Halo, with the Flood being able to distribute all information instantaneously once they're all working from a Gravemind.
6. No, more akin to having somebody remotely controlling all of those machines with root-level access, and in the case of AIs/"Machine Spirits" essentially convincing them they're best to work on their side, while also stripmining them for information.
7. AdMech is going to have issues with every single system that uses a machine spirit or a servitor, because The Flood can readily interface with it. Once that happens, anything that connects to a compromised machine is fucked. Not to mention that given the reliance of the AdMech on people actually operating stuff, people being influenced by their psychic bullshit wave can do shit for them without anybody even knowing the Flood are at work. Augmented people are in the same boat.
Nids are biological, so they get to deal with the biological elements of The Flood. The Necrons are in for a really bad time, since they are connected to each other somehow.