Hello,
What is the start address and size of OTP in S32K312?
0x1B000000 and 8kB is size.
+
I would like to store a 4K key into this memory . Is it possible.
You can store it here, at first free available address. Depending on your DCF usage

How do I ensure that this region is locked and read -only?
It is by default from NXP facotry.
Is this region accessible from any Flash Bank ?
Yes
Also, are there any dependencies in terms of HSE operations when HSE is enabled on this region.
No, HSE will use only blocks defined for HSE usage. There is no choice.
Best regards,
Peter