#3. People who receive the key from #1 (in the future).

This may be next year's elected official, or the official elected in 10 years, or the spy who will work in the office 7 years and 2 months from now. Can you guarantee the intentions and actions of every single future government employee and official? This, in my opinion, is the biggest threat!

You can't even guarantee the intentions and actions of the government in its entirety, good governments have historically been succeeded by bad ones in roughly the same way that rain eventually follows sunshine and the other way around. To assume infinite stability of government is a major mistake.

The second occurrence is not only inevitable, it refutes the notion of a door in the first place, which is supposed to be fully opened at some stage for long periods, not temporarily opened in-case-of-emergency, or half-shut just-in-case.

No, they do think and know (2) will happen, and are probably hoping it does, as they realise that when it happens they can use it as an argument as to why they need MORE surveillance powers and MORE backdoors because "the bad guys" are STILL getting through, so more surveillance powers will be needed to keep you safe from the evil without, and to help you keep your British values good and strong.

Playing Devil's Advocate, couldn't the company require something signed with their private key to open the back door?

Apple (for example) would already be in serious trouble is someone got their private keys, they use them to sign OS updates for example. If they could generate a device-specific signed cookie to say "open up", what harm would that do?

These things also have a habit of growing arms and legs - so another issue is the number of people with the key. I can almost guarantee that at some point the number of people with the key will grow (police forces, councils...)

Yet another issue is oversight of those with the key (who watches the watchers)

They know it'll happen. The terrible part is, they don't care. It's a risk they're willing to take to acquire the power they're after.

...and yet we all still have door locks?

I don't think your analogy is going to help convince people that the policy is bad.

And people with a key could take your data and sell it.