I'm not a designer, so this is just my opinion:
Cards have to not have sub-types when facedown, because there is no way for the opponent to check them, and that makes it possible to both cheat and be mistaken. It's an unwritten rule for most games to not do stuff like this. It's why in say MtG when you search your library for any card you just grab one and don't reveal it, but when you search for a Sorcery you have to reveal it to prove it's a sorcery.
Sesyrix, Sectarus, Cassiel and Naiya all break this unwritten design rule, because they have to. Enchantments have to stay unrevealed and unknown to an opponent before they are revealed. The lesser of two evils therefore is to take that bit on trust from the player that they got it right, figuring that it will later get revealed at some point. Those cards would would almost certainly be banned from tournament play for a big tournament game like MtG, but MW is more casual, and the decision was obviously that they were happy with this compromise.
As to where it comes from that Facedown enchantments have no subtype - it's been ruled so here on these boards by Laddinface (Designer) and it's in a collated set of his rulings that are not yet in official documents by Exid. You'll find them both on these boards (the Exid list much more easily!).
Do face down, spellbound cards, maintain their spell type and level and traits? If not, is it legal bind any old thing to a wand as long as i don't reveal?
Logically yes they would lose their SUBTYPES (you are confusing these with Traits), not that this has any relevance for cards at present.
It is
not legal to bind the wrong card to a wand however, but yes it would be undetected if never revealed, which is the same problem that the above Familiars present.
