Ta. I slept on it, saw no additional thoughts herein [nor had i any], so grit my teeth & initiated the reboot of that VM, fearing the worst. No rescue screen, but no boot menu either, just straight into Manjaro [which itself is fine as usual]. Therein, i repeated the grub update, & happily this time, it worked correctly/fully, finding my Fedora 2nd-boot [which per my earlier post, had failed before]:
[k@k-VM-ManjaroFedora ~]$ sudo update-grub
[sudo] password for k:
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-5.1-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.1-x86_64.img
Found initrd fallback image: /boot/initramfs-5.1-x86_64-fallback.img
Found linux image: /boot/vmlinuz-5.0-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.0-x86_64.img
Found initrd fallback image: /boot/initramfs-5.0-x86_64-fallback.img
Found linux image: /boot/vmlinuz-4.19-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-4.19-x86_64.img
Found initrd fallback image: /boot/initramfs-4.19-x86_64-fallback.img
Found Fedora 30 (Thirty) on /dev/sdb2
Found memtest86+ image: /boot/memtest86+/memtest.bin
Did another VM reboot, & indeed now have the full boot menu back. Phew.
Given this VM was the dual-boot testbed i established weeks ago prior to then successfully deploying in Tower for real, this VM's update hiccup alarmed me & made me hold back from doing the update in real Tower. Now it seems i can proceed...
Additional info [irrelevant for all Manjaroos unless also using Fedora30, but provided fyi].
Upon reflection, i now strongly suspect that this storm in my teacup was caused by --surprise surprise-- my own usual ham-fisted incompetence. Unlike the magnificent Manjaro kernel update/install process that automatically includes the update-grub routine, in F30 when one installs a new kernel, one must remember afterwards to manually run the update command, before rebooting, otherwise said new kernel/s remain invisible in the boot menu. Furthermore, the choice of specific command is dependent on whether one's F30 installation was UEFI or legacy. In the case of my VM, last night i muddled myself & mistakenly ran the F30 command for UEFI, when in fact i should have done the GRUB2 one. Ie, last night i ran
sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg, whereas this morning i corrected my error via
sudo grub2-mkconfig -o /boot/grub2/grub.cfg.
Such a silly sausage.