Opened my "0000 - Byleth♂.character" file using HxD, and lo and behold.
0x00-0x17 are items, in the XXXX YYYY (where XXXX is the item ID, which matches the IDs given via the Items tab, and YYYY is durability) format. 0x18 is the terminator value.
Magic, I believe, is located at 0xA0 - my Lv. 99 Byleth, who had learned all his magic, has Fire/Thunder/Bolganone/Ragnarok/BLANK/Heal/Nosferatu/Recover/BLANK/Aura.
These spells have the IDs 00/03/01/02, 18/1C/19, 1E.
0xA0 contains 00 03 01 02 26 18 1C 19 26 1E - where "26" must be the BLANKs.
The number of uses (which is quite a silly mechanic, if I may say so; completely cripples Warlocks and their ilk, and replaces a better system seen in previous games, where tomes had a couple tens of uses and magic users could carry several tomes of the same spell. Thank Sothis for being able to hack the enemy-only Infinite Magic ability onto playable units) is located at 0x94-0x9E.
Since I had zeroed all of my Skill EXP before exporting my Byleth file, I do not currently know where that is.
Correction: I believe the Skill levels are 0x88-0x92, since my Byleth has 7 S ranks, 1 S+, and 3 S ranks, in that order, and 0x88-0x92 contains seven 0As, one 0B, and three 0As. That can't be a coincidence. Still don't know where the actual Skill EXP is, though, since I did indeed zero it all out before exporting - oops.
Class levels are at the end of the file - 0x1E7 onwards - looks like. Class EXP is somewhere in the 0x130-0x140 area; I somehow have 2 EXP for Sniper, Grappler and Bishop, but they're far from the first few classes.