Using iPXE on installed on local drive to boot Windows from local drive

[FreeMinded]

Hi iPXE community

I'm pretty new to iPXE, so please bear with me.

We are trying to use iPXE to achieve the following:
In a distributed setup we want the clients (Desktops or Notebooks) to ask a defined server what to boot. In the normal case this would just be to boot Windows from the local hard drive. But when necessary the server can instruct the client to boot a specified image from the network (i.e. to reinstall Windows).
For security reason we do not want to use regular PXE booting from the network as we can not control the environment and PXE would open a potential attack vector.

Currently we chainload iPXE from Grub both of which are installed on the local drive. iPXE then contacts the server and gets a ipxe script file back. So far so good.

What does not work yet is to boot Windows from the second partition of the same local drive. Using
sanboot --no-describe --drive 0x80
Grub loads again and we loop.

Using
exit
ends up in a "No bootable device" error.

Questions:
How can I boot a specific partition from iPXE?
Is there a better way to accomplish what we are trying to do?
Is there a way to use timeouts in case the server does not answer (clients should then always boot Windows from the local drive)?

Any help and hints are highly appreciated!
Regards,
FreeMinded