The encryption only has one way through, with the passkey. So the encryption works and this is good. No backdoor there.
The 10 try deletion is a completely separate system from the encryption. If you stop it from deleting, the encryption is still intact. No one will be able stop this from happening short of Apple taking action or divine intervention, so there is no backdoor here.
What you seem to be pointing out is a philosophical point, about if in the future Apple designs software to backdoor your phone, then there is the potential for a backdoor right now, therefore, there is a backdoor right now. This is false. There is no backdoor. Apple, being god in this case, could make a backdoor. Apple, or god (which ever you prefer) didn't make a backdoor, so there is no backdoor.
Not true. To say something has a "back door" is to imply security was deliberately compromised by the manufacturer, whereas a vulnerability is a security weakness in the the architecture or implementation.
I get what you're saying but it would have made more sense had Apple not added the Secure Enclave to newer iPhones. I.e. they tried to make it more secure, so I have to conclude that they had tried their best with the older versions, too.
A simple flow chart or state diagram would show the vulnerability. I'm not saying it was intentional, but I believe the backdoor label can be applied to accidental entrances as well.
The bottom line is that Apple produced a device whose security features could be circumvented.
If the "10 false codes and the device is bricked" can be circumvented, there is a backdoor.