Hi,
-the first part: is it possible to switch from reserve wan (mobile, for example) to main wan (wire, for example) when icmp check on main wan succesfully passed again? if yes - how it can be configured?
It is done by default, router check when Main WAN start working again and switch back from backup wan.
- the second part: is it possible to start l2tp connection (rut device as a client) only when wan switched from wire to mobile, and break the connection when wan switched back to wire again? if yes - how it can be configured?
Router do not have such feature. But you could try to block via Firewall access to L2TP server from source IP (Wired WAN IP).