If the altitude is lowered on the 29's, then more options become available. You could go one further and only have single B-29's. The problem then becomes finding enough pilots to make it viable. Also, from what I understand the Ki-84 is actually not a bad plane at that altitude (20k-ish)?
But the 43 will probably still struggle a bit. I haven't flown her very much so I cannot say for sure.
I've only ever flown the N1K1 like 10 times, ever. I cannot say what she's capable of at any altitude (other than being on fire, since that is how almost all of my sorties ended in her
). Ultimately it's the speed of the 29 that will prove a challenge as well. Even at cruise she still moves fairly quick.
Too slow and too low will kinda ruin the event. But obviously we do not have a Japanese plane that can adequately intercept the B-29 flying at her historical altitude and speeds. It's like the time when A6M's were tasked with intercepting B-17's during an FSO I was apart of years ago. We only have the G's and even though the 17's were singles, they simply outran the A6M's. I don't think the group tasked with intercepting them ever engaged them or their escort.
Definitely a delicate balancing act when it comes to the B-29.