Hi,
Below will be some comments on your open points:
1 > HSE Firmware Security Installer Source Code
[DA]: Can you help us elaborate more on regards of the SW you are looking for? Where is it referenced? We do understand that on regards of HSE-FW, no source code is provided. We do apologize.
2 > The HSE Firmware Reference Manual
[DA]: It seems to be that there is a problem at this moment under the S32G2 product page for retrieving the HSE-FW Reference Manual, we do apologize.
If your account is correctly enabled, you should be able to get the HSE-FW RM with the following steps:

- Under the new page, select the "File" option and search for "HSE-H/M":

- If available to your account, you should be able to see the HSE-FW RM. If not, then it might be that your account is not correctly enabled to access the HSE documentation. For that, help us contact your local NXP FAE/DFAE/representative.
Please, let us know.