How can SPV nodes trust received Open Assets?
You can't trust pure SPV for any Bitcoin 2.0 protocols, because of the 'metadata faking' issue that you outline. You have a few options:


Accept you need to run a full node to backtrack through transactions. (Though note that this will not scale well anyway for popular assets.)
Use a blockchain that has native support for assets, like Sidechain Elements or MultiChain.
Use some kind of external service to track assets, like the CoinPrism API for Open Assets or CoinSpark's federated asset tracking model.