If I remember correctly the gateways detect whether NAT exists between them in IKEv1 main mode packets 3 and 4, then NAT-T on UDP 4500 starts at IKEv1 packet 5 if needed. So even if NAT-T is forced from the start I'm pretty sure IKEv1 will still use UDP 500 in main mode packets 1-4 which would be expected behavior. If you are failing out after IKEv1 main mode packet 2 it is just a settings mismatch (encryption, hashing, etc.) that you need to correct.
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