This kernel is end of life, support will end soon, better switch to another kernel and remove 5.7
Yes, why not.
Probably not directly, but you should better not share your EFI partitions, create a separate EFI partition for your Linux systems.
Yes, but you need to properly mount a ntfs partition if you want to use it for both systems, and it’s absolutely essential always to fully shutdown Windoze, don’t use Fastboot or any other hibernate features if you want to avoid corrupt shared file systems.
If all systems are installed in the same boot mode and all disks are the partition mode, i.e. UEFI/gpt vs BIOS/mbr you should be able to boot all OS from grub’s boot menu. The disk with Manjaro’s EFI must be the one to boot from your firmware.