1. Why Japan and not another country?It's not really too far-fetched to speculate on, though. The Japan software charts are always dominated by Switch (and specifically, Nintendo 1P, aside Minecraft and maybe a random Square-Enix game here or there) titles. I personally thought we'd see more PS4/5 titles regularly in the Top 10 after FF XVI's release, at least 2-3 PS titles in Top 10 even if on the lower half of the chart. That's what I expected.
Instead such hasn't really panned out. And yeah, we do have data from the ransomware hack showing digital skew for Japan is lower than many other markets, so even if accounting for F2P usage (which in order to verify, we'd have to get player data statistics for games like Apex Legends or Geshin Impact on console in Japan) that doesn't 100% explain hardware/software discrepancies.
So, are there some portions of PS5s in Japan being exported to other territories in Asia (or markets nearby like Australia or even Russia where IIRC PS5s are not being officially sold due to the war)? Probably. What percentage? Anyone's guess. But it's very likely beyond the margin of error (aka > 3-4%). It sucks that bad faith actors and fanboys have latched onto that idea as an anti-PlayStation narrative, but their stupidity doesn't in itself completely invalidate the theory itself.
2. If it's to do with currency, are we sure people aren't just importing switch games from Japan thereby inflating the software sales there?