Yes, however inconvenient it may be, at least that option exists.
It might have. That’s only a matter of configuration, and I suppose install-grub would have reset that to the defaults.
To change it, the easiest is to edit /etc/default/grub;
To have the Grub menu display and the last booted OS set as Grub’s default boot selection, ensure that these lines are uncommented; change as needed, but this works for me:
GRUB_DEFAULT=saved
GRUB_TIMEOUT=5
GRUB_TIMEOUT_STYLE=menu
GRUB_SAVEDEFAULT=true
And then, uncomment this line (leaving it exactly as shown) to ensure other loaders are detected;
GRUB_DISABLE_OS_PROBER=false
Close and save.
Next, update the Grub configuration:
sudo update-grub
Reboot.
The answers to these questions should now have been made obvious. 
I hope this helps.
Another option exists to install the refind boot manager;
By choosing refind as the initial boot loader, other OS boot loaders – in your case Manjaro and Windows – are able to be chainloaded.
refind also has a welcome feature of being able to bypass Grub and boot the kernel stub directly, which is convenient in the case that a problem with Grub prevents booting – one can still boot to the Linux system and work the problem in relative comfort.
I mention it briefly in a related tutorial.
This does take some manual configuration, so I’d suggest spending the time needed to research, before taking this route.
Regards.