Hey there. I'm a 4.4k offlaner and Clockwerk is my most successful hero at about 62% winrate.
1- Why doesn't offlane work for you? Clockwerk, at least in this current patch, is much more suited to the offlane than the midlane. If you're uncomfortable with the offlane I can give you tips on that if you want.
3- Clockwerk is one of the most versatile heroes in the game. Many items can be considered core, but honestly every game is different. Blademail is definitely a strong item but you shouldn't be building it every game, and the same can be said about Aghs. A piece of advice I would give you is to try to match your item build to each game individually. Considering phase/BM/Aghs core, you'd have to spend about 7.5k gold before you can move onto other items that may be better suited to the game, like Force or Urn.
4- Clockwerk is a very underrated farmer when he has a bottle. Throughout the midgame, if the enemy is playing too safe, I pop into the hard camp and use my Q to farm it up while flaring lanes for more CS. Don't be afraid to use your Battery Assault on creeps, it can really contribute towards your GPM/XPM.
5- You're pretty much right. Both heroes can snowball quite well throughout the midgame and can cause people to play very defensively.
6- Regarding point 3 about items builds - you can increase that list even further with smart items. If you get a good start and you're facing heroes like Juggernaut/Lifestealer, you can convert your early lead into an Orchid that will COMPLETELY shut down those heroes. Those two heroes rarely build BKB, and because their magic-immunity is skill-based, Orchid will make it so the only thing you have to fear is their right-clicks (which is where Blademail shines). There are other items and hero interactions but I can't list them all in one post.
7- Clockwerk is the definition of creating space. Not only that, but he fills the role of both initiator and counter-initiator. He's fantastic when paired with long-ranged DPS/Nukers like Sniper or Skywrath. Spectre is a really strong combo with Clockwerk because after he hooks in and creates chaos, Spectre can haunt into the fight and deal a lot of damage while the enemy team focuses you. Invoker is another good pick for early game ganks with Sunstrike while your enemy is cogged with you. Mainly though, Clockwerk can work in most lineups. He's viable in literally any MMR range.
I'm going to classes now but when I come back I'll gladly give you some pointers like how to block the Dire creepwave in the offlane with your cogs to have complete control over lane equilibrium, and other fancy tricks like that. But for now, I hope I helped!