I will try as you advised probably tomorrow. But I can see the potential problem here. If my memory serves me I was trying this before posting here.
I'm not sure if L2tp client of RUTX11 assigns WAN IP address to Mobile Interface. L2tp just makes the point-to-point connection, doesn't it? It seems to me L2tp client was able to connect only when Network > Interfaces > Mob1S1A1 was configure in NAT mode. This is how L2tp was able to reach and connect to LNS endpoint, to establish tunnel and to get Static public IP on this, let's say, L2tp interface.
When Network > Interfaces > Mob1S1A1 was set in Passthrough mode then an internal IP 10.x.x.x of Mobile Interface has been assigned to the firewall. In the same time, L2tp connection were running independently on RUTX11.
Your idea should work in case there is ppp over Mobile Interface to be available. This is how l2tp can run within Mobile Interface. As far as I know, it is not implemented in RUT and this makes me sad: 1, 2.
This is why the idea for know to find out how to passthrough the l2tp connection itself? The other idea is if that is possible to make an new interface dedicated to l2tp connection and passthrough this new interface?
My firewall support L2tp VPN and I was relying on this idea. Unfortunately, this doesn't work as it's intended to work. I have raised the support ticket with the vendor and the investigation is going for more than a month already. This is why I seeking for some workaround.