site to site VPN with overlapping address pools

Hi all :)

I work for an IT service provider, we want to establish site to site Ipsec vpn connections with a set of our clients. we use a fortigate box placed behind a DSL modem. Network adressing is as following:

subnet behind the modem: 192.168.1.0/24

subnet behind the fortigate box: 192.168.30.0/24

my first question is: since the vpn source is behind a modem, how can I pass all vpn trafic from the modem to the fortigate? do I need to put the modem in bridge mode or is it possible to use port forwarding with IPsec trafic?

my second question: we found ourselves in a situation where a client uses the same subnet IP addressing, is there a way to establish the vpn even between overlapping subnet addresses?

Thank you all