ITYM "WI the Striking Force was sighted a few days before the Pearl Harbor attack?" The "Combined Fleet" was the term for all the seagoing vessels of the IJN: the destroyers, cruisers, battleships, and carriers. It excluded minesweepers, patrol craft, escorts, submarines, tenders, and training vessels. If the entire Combined Fleet was somewhere to be spotted, that's weird.
The Japanese expected to have operational surprise (they achieved tactical surprise, which was a bonus). That is, they expected to be spotted a few hours to a day before the attack, which would not allow the US forces to get fully ready for battle. (Consider what the US forces would need to do: get the report to all the relevant commanders, get everyone out of bed, get the command staffs up to speed, get all personnel out on liberty back to their posts, get steam up in all warships, get all planes armed and fueled for takeoff, get the fleet out to sea... All from a "standing start", i.e. peacetime conditions. Thus even with several hours warning, the US forces would be at a disadvantage.)
It has been written in this thread that Nagumo's orders were to turn back if sighted more than 24 hours before H-Hour, because that much warning would lose operational surprise. Otherwise, press on.
If per tbhe OP Nagumo turns back on say 4 December 1941... ISTM that Japan is already committed to war with the US and Britain. So the "Southern Operation" continues. Lots of issues to discuss but I have no time now.
Here's one:
The Japanese landed in Thai territory at the start of the Malaya operation. Thai forces opposed the landings, and inflicted hundreds of casualties before their government ordered them to stand down. If the British have two days warning, what do they say to Thailand, what does Thailand do? Do the British execute Operation MATADOR (move forces across the border to the landing sites)?