Would you even acknowledge? The data is worthless after one round-trip, so you wouldn't send it twice. The receiver just needs to make sure that they read information correctly and can deal with losses.
Right. A shortwave link would be used in addition to a fiber link, not as a replacement for it. Assuming ACKs aren't latency sensitive, they could flow back over the fiber.
No, your trading engine is on the other end, the link only sends prices. Let's assume you want to trade on a future in Chicago that's based on London prices (e.g. a world wide equity index). You know Chicago prices and NYC prices and what people in Chicago think London prices are. If you then get from London a price indication that this has changed, you can quickly trade against it to make money. The London link never needs to know what happened, they just send prices that could be interesting. All logic sits at the exchange where trades happen since that's dependent on local prices at time of execution.
SOH<header data>EOH STX<data>ETX CRC EOT
Then the receiver would send an ACK or NACK.
Repeat as necessary.