I've tried the suggestion above and it unfortunately does not work. However, there is another trick you can use. The initial packet which comes from the remote peer contains, among other things, the IKE version, and at least in 6.43rc12 where I currently test it, the IKE version (which can be derived from the exchange mode) is one of the criteria used to choose a peer. So if you configure the site2site VPN between A and C to use IKEv2, the peer with address=C and exchange-mode=ike2 on A will be ignored when handling initial packets from the l2tp-ipsec clients indicating IKEv1, so for these connection attempts, the peer with address=0.0.0.0/0 and exchange-mode=main-l2tp will be chosen.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace each occurrence of any public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.