Read this:
https://multibit.org/help/v0.5/help_support_sentBitcoinFromMultibitButTheyAreStuck.html

There are two main reasons why a transaction gets stuck on "Seen by one peer" and does not propagate through the Bitcoin network. These are:
You lost your network connection and the transaction did not get sent successfully to the peer (did not broadcast)
The peer received the transaction but "did not like it" (did not relay)

There are various reasons why a peer does not relay a transaction. In a rough order of frequency they are:

There are lots of tiny inputs (under 5432 satoshis) in your wallet and the transaction fires the anti-spam rules.
The fee is too small and the peer chooses not to relay it.
The transaction is a double spend. Doubles spends aren't relayed through the network.

You can remove the transaction that never propagated by doing a 'Tools | Reset Block chain and Transactions'.

IMO this is where your problem lies. I'd reset and try again until it gets accepted. I see no other way since you can't upgrade to a newer version and can't increase the fee. Once you get the transaction to appear in the blockchain you can accelerate it, but without this happening there's not much you can do since it's still stuck in your wallet.