Starting undionly.efi or snponly.efi from the linux command

[NiKiZe]

You can use ipxe.lkrn and start that from syslinux or grub in legacy mode yes.
Booting efi binaries from grub seems flaky,
you could put ipxe.efi on the efi partition and add a boot entry to it into the firmware, meaning that it can be selected before grub or any other bootloader.
(You have to remember that Legacy BIOS and EFI boots are different.)
You can not enter efi mode from legacy mode, and you can not enter legacy mode from efi userspace - don't mix this up!

undionly is only usable if there is an existing nic driver that can be used - which in most cases are in the NIC ROM
Same for snp devices.