Re the 500 Hz "error" when using "Split" or "Fake it".
The purpose of those two settings is to make sure that the FT8/FT4 audio signal stays between 1500 and 2000 Hz (to avoid the risk of intermodulation in the audio passband). If you select an audio TX frequency (the red "goalpost" in the waterfall window) between 1000 and 1499 Hz, say 1200 Hz, you would expect your signal to be transmitted on e.g. 14,075.2 kHz. To transmit an audio signal in the 1500-2000 Hz sweetspot, the USB "carrier" or dial frequency is therefore moved down from e.g. 14,074 to 14,073.5 kHz. Your audio signal will be 1700 Hz, right in the 1500-2000 Hz sweetspot. Your signal will be transmitted on 14,075.2 kHz, which is the sum of 14,073.5 kHz + 1700 Hz.
If you use the "None" split setting, you will transmit a 1200 Hz audio signal on the 14,074 kHz dial/"carrier" frequency.
Ref. the User Guide section 4.2.