Does the protocol implement any kind of negotiation (ciphers, ...) ? if not, how would you handle future type of attacks against the then hardwired constructions ?

I fully agreed that being in-kernel is the right choice for performance, but the chosen constructs excludes the possibility of using any type of existing crypto hardware accelerator that shines in the IPSEC use-case (cache cold data == no cache flush overhead, fully async processing of packets with DMA chaining). Time to start lobbying SOC vendors :)