Personally, I would immediately book the cheapest, refundable business class tickets on a convenient equivalent routing.

Point out that getting refundable tickets will allow them the opportunity to sort things out while also guaranteeing you the travel you paid for, should they fail to perform.

Point out that by purchasing the cheapest available tickets you're minimizing your loss due to any failure to perform the contract on their part.

The difference between the price you paid bmi and the price of those tickets is the amount of the damages that a court would award for breach of contract.

Even if you can't afford it, you can always refund the tickets up to the time of departure if you want to make some other arrangement. It would indicate that you were serious though.