How do I install macOS Sierra now after all these issues?
I've been looking over your other questions and the whole history of your situation and feel like we're missing something here. Not blaming you, I just think something unbeknown has happened (e.g. a power failure at some point you're not aware of that has caused some corruption).

So, I'm suggesting you do as follows:


Start your iMac in Target Disk Mode
Connect it to your MBP (make sure the MBP is connected to AC power)
Run Disk Utility from the MBP to erase the iMac HD (no need to do another secure erase) and also run First Aid
Assuming you don't encounter any errors, do a fresh download of the install macOS Sierra app from the Mac App Store (i.e. do not run the installer from the USB installer you created, but download a fresh copy from the App Store directly to your MBP)
Now run the installer from the MBP
Follow the steps to install macOS Sierra (ensure you select the iMac's HD to install it on)


Let me know if you run into any problems, but hopefully this will resolve the issue once and for all as it eliminates a number of possible problems.