Some questions about VPN

Routers VPN, DNS, Leaks
1
  1. Is it possible to set up two different VPN? One per guest another one for main?
  2. Can VPN host (for example NordVPN) access my LAN?
  3. How can I prevent real IP leaking? Any additional configuration needed?
2

No. Not in GL firmware.
You can set up OpenWrt PBR if needed. But it is better to stay with the GL features if you need support.

Mostly no if you only use client mode. When you use server mode it is possible, see: VPN Dashboard - GL.iNet Router Docs 4

You should check your setup by using various "IP leak" webservices like ipleak.com, ipleak.net, My IP Address - BrowserLeaks and so on.