I put the USB stick back to state described in
#14.
I added additional menuentry as above in #17, except that I changed menuentry name to "Porteus-LXDE-v5.0-x86_64 (2)".
Grub boots OKs.
When selecting the new menu entry "Porteus-LXDE-v5.0-x86_64 (2)", the result is the following:
b4: hd0,msdos1
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/ufs2.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/ufs1_be.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/ufs1.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/udf.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/tar.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/sfs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/romfs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/reiserfs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/procfs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/odc.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/nilfs2.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/newc.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/minix_be.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/minix3_be.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/minix3.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/minix2_be.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/minix2.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/minix.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/jfs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/hfs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/f2fs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/exfat.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/cpio_be.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/cpio.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/cbfs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/bfs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/afs.mod.
error: Secure Boot forbids loading module from (hd0,msdos1)/boot/grub/x86_64-efi/affs.mod.
error: no such device: /boot/syslinux/vmlinuz.
aft: hd0,msdos1
error: file '/boot/syslinux/vmlinuz' not found.
error: you need to load the kernel first.
Press any key to continue...
I took a photo of the boot screen and wrote the text above based on the photo, so there might be some spelling errors.