Hi!

Welcome to FT. I suggest you send a PM to the moderator to move this to the British Airways forum where you will get a better response rate.

To be honest, your question is pretty muddled, and I'm not sure of what you are asking.

This is complex, as the BA rules have changed significantly. It is now distanced based, and you would have to ask BA (or ask in the BA forum) how many miles it would be. There might be a new mileage rate for TPA-China. Even if it's the same, the last leg would be treated as a separate flight, so it would likely be more miles no matter what. Additionally, you would have to pay the change fee.

Since you clear customs in ORD, you could probably not take the flight, but you wouldn't get anything back.

If you are traveling domestically on an international ticket from China, I don't think you will pay bag fees, but just showing them a separate ticket won't help you, it would have to be one ticket.