S/PDIF: TTL to Coax Converter
1) It is a CMOS inverter, it works poorly when supply is 5V but signal input is not 5V but 3.3V. Another thing could be output drive strength. It is also not ideal to parallel outputs - at least not directly without series resistors. 72 ohms is of course not perfect but within 5% so that should not be an issue.

2) No, not really. But the resistances would suggest it does not use 5V supply, but perhaps 3.3V. If you have a reference design BOM you would know the inverter type.

3) Three gates in parallel can drive more current than one or two.