They could not. In the 1860 election Lincoln won 15 states with absolute majorities for a total of 173 of his 180 electoral votes. Only 152 were needed to win.
Sure they could have !
You are just combining the actual votes received.
You are not looking at what the effects of a combined and unified campaign would have had on the votes and the election outcome.
They lost a massive amount of campaign time and good will just by having a failed convention!
Both factions had to hold a second convention to select their nominees.
That would sap a lot of energy, money and time out of their respective campaigns.