How can I safely test custom ROMs on an obscure device?

I have a ZTE Blade A1 with pre-installed malware. I'm only able to identify and remove some of it. So, I want to delete everything on the phone and install a custom ROM. Unfortunately, I cannot find any ROMs which explicitly support the ZTE Blade A1.

I've decided to try putting CyanogenMod on the device (or possibly something else), as it does support older versions of the ZTE Blade. My plan is as follows:

1. Create a backup of the existing ROM using ROM Manager


2. Run the device in recovery mode (using the default Android system recovery - CWM or TWRP do not explicitly support the Blade A1, so I won't risk using them)


3. Use the fastboot commands fastboot -w and fastboot update rom.zip to install the custom ROM


4. Restart the phone, and if it has stopped working, repeat steps 2 - 3 using the backed-up ROM from step 1

I've read up on using custom ROMs, but have found a lot of conflicting information. So, my question is: Will this replace the pre-installed OS with a custom ROM? Is there a risk the phone won't be restored to its original state?

Update: I haven't been able to confirm whether the phone can be irreversibly bricked by installing a custom ROM. However, if anyone comes across this question wanting to remove malware from the ZTE Blade A1, I have posted a solution here.