OK, so I solved everything. For future viewers, I'll explain what solutions I used:
Problem: Wanted to Install W7 on a new laptop.
Solution: Don't. It's better of to get used to W10 and solve its problems.
P: Wanted to boot from external HDD
S: startup settings - uefi - secure boot = disabled, boot = legacy, boot order = external HDD
Problem: Deleted system partition when my only hope was a W10 ISO in an external HDD
P: Once booted from external HDD, I wanted to copy the OS to a local HDD
S: Paragon Hard Disk Manager (migrate OS)
P: Internal HDD was read-only
S: in DISKPART, attributes disk clear readonly
P: Couldn't boot to uefi settings to change the boot order again
S: Internal OS disk needs to be GPT and not MBR.
In DISKPART: convert gpt
Here are the relevant DISKPART commands I used. DON'T USE THEM ALL, it's just a collection:
Hope that helps.
select disk 
select volume 
create partition primary
select partition 1
active (only works when disk is MBR)
format fs=ntfs quick
convert mbr / gpt
attributes disk clear readonly