A last question: It appears to be safe to delete user entries from table user for users who haven't made edits (can you confirm this ?)
No. Even if they haven't made edits, moves, deletes, protections, and don't have any watchlist entries, even then there could be entries for them in 'user_groups' and/or 'user_newtalk'. You're safe only if you remove all of these, and I'm not making any guarantees that I haven't forgotten any ;-).
Timwi