Why sync needs restart?
It doen't normally however you might have some issues going on your ESXi hosts which a restart may resolve.
And why 5min?
The vDS information is stored locally and synchronized with vCenter. It makes no sense to do this continuously so having a periodic sync period to make sure the vDS information is correct make sense
Sounds like Windows 3.1
No comment.
Looks not very possible. First there are lots of VMs with vNIC-s, with share settings and then I must manually set them all back to right place. Ok, I can use profile save, but Im not 100% sure. Because there runs also NSX, NSX manager, controller, 2x edge.
Do you not have another ESXi host that you can vMotion these VMs to so that you can at least restart the affected host?
Reference added: https://kb.vmware.com/s/article/2042692