[Ethereum] Performing a smart escrow with ERC20 tokens

.send() function sends Ethereum from contract balance, not tokens. So you need to call token transfer function.

Also I advise you to always you send() function within require() because send() doesn't throw exception and your debug process could be harder.

Additionally, why does the contract not have to sign the transaction as a user would?

Because contract can't execute it's own functions by itself. All functions can be triggered only when real user send signed transaction with correct data field (contains information about calling function and arguments) to Ethereum network.

1) Depends on what you mean with a "trigger". Smart contracts can never do anything without someone/something issuing them a transaction. So contracts can't "monitor" things by themselves or run in the background in that sense. Otherwise, yes, a smart contract can transfer tokens to another contract/address.

2) Because all action is initiated through a transaction, the original transaction sender is the "signer". The one who issues the original transaction also has to pay (with gas) for all the transactions that occur in the possible chain of contracts.

EDIT After the asker added more information about the problem:

Sending tokens to another address is done by calling the transfer method in the ERC20 token contract. That method should actually just modify the contract A's inner state, so transferring tokens from contract B only requires B to call A's transfer function. B does not need to "send" any tokens itself - it only needs to call the transfer function to change the ownership of said tokens.