A bit more on my research - even if nobody comes by with the answers, doesn't mean I shouldn't detail the ones I've discovered.
As detailed in
http://www.h5.dion.ne.jp/~rimemoon/zaurus/memo_006.htm (I wish I could understand more of it, but all of my japanese-speaking friends are not available at the moment) apparently, you can recover the kernel files referred to in NAND Checksum with the D+M service menu, and not using the blind rom method described in
http://pocketworkstation.org/files/recover/ recovery kits, for which a CF card with the same files will be necessary.
Unfortunately this page does not provide any links to files which would definitely work in a 860, or I'd go for it.
Reading more into it, the failed checksum of KERNEL1 means that the main linux kernel that runs the boot process is screwed, but the maintenance kernel KERNEL2 is still intact which allows me access to the service menu. I do wonder what are the ROFLSYS and INFO files for though.
Reading yet more into it, it appears that erasing NAND will not cause ill effects IF it is restored from backup immediately afterwards, since the service menu program will keep running in RAM. I'm not very certain of that interpretation though, so I'll have to leave this as the last resort.
More interesting news. According to
http://externe.net/zaurus/forum/viewtopic.php?t=1297 the recovery kits from pocketworkstation.org do NOT work for 860, since they were assembled from earlier versions and don't actually match.
That makes me think NAND backups from other sources for the same model actually have a chance of solving my problem, though.
After restoring from a japanese NAND backup, the machine still did not boot. NAND Checksum displays a NG on PARAM partition, but no clue as to why would this prevent it from booting, or what to try next. Restoring with my old image after that still produces no improvement.