1) There is no practical maximum level. It is probably limited by the size and type of the variable, so at least 127 and most likely much higher.
2) Attaching an item to a legion/PoP has no effect on the item. However, if the item grants bonus levels or loyalty, the legion will benefit from the bonus (so it can be useful to pilfer the items before bribing the legion).
2b) As Chaosguy mentioned, opponents can't normally see the contents of your vault and relic chamber, so there is an additional step (and a bit of warning to you) before they can pilfer them. I would also assume you can hide items on your legions when the legions themselves are cloaked (I haven't verified that though). I normally keep relics in my ritual chamber unless I either need the ritual slot or don't care about the item. The main exception are +prestige items which only give prestige when attached to a PoP.