Unfortunately I can't answer your question about 2p MP12s. If it falls through, or is cheaper, another solution you could try would be Dreamcast-compatible PCBs through maple adapters (Mitsurugi-w, MisterAddons, and TR Fightsticks sell them).
Overall, it's very unfortunate, but it doesn't seem anyone has published latency test numbers for the various JVS solutions, some of which have the reputation to be laggy, and some have interpreted it as the games themselves being laggy (look at the discourse around SFZ3Upper). I have heard that this is fundamentally a limitation of Naomi's low JVS polling rate, but again, no one has tested the various solutions and posted lag numbers with a transparent measurement methodology. Because of that we don't know exactly how the lag compares between the Capcom I/O, Sega I/O, MP07-IONA-US, MP12-IONA-SB, and OpenJVS. I also don't know the lag of the Maple adapters, which could possibly be lower or higher than JVS as they (to my knowledge) bypass the JVS I/O. If anyone has proper lag testing equipment and a bunch of the hardware I listed, they'd be filling in a big knowledge gap in the Naomi community