1) I assume that here by buffer count, you mean the DMA Buffer count in FX3. Assume that the size of each buffer in FX3 is 1 KB. You tatally have 1KB * 6 = 6 KB of buffer space.
The number of bytes you transfer in XferData will give the number of buffers you fill. Assume if you send 5000 bytes, you will get 5 Buffers filled (4 full buffers and one buffer partially filled but wrapped)
2) This has not been characterized, But you can call Xferdata continuously in yous host application, without any delay inbetween. It would support the USB datarate.
3) The size of the DMA in FX3 really matters. You need to make sure that DMA Buffer Size * DMA Buffer Count is less than around 250 KB. For exact details, please refer the TRM.
4) Xferdata returns false and the transfer is Nacked.