The allowance to say havdala until Tuesday is primarily for those who forgot to say it on motzei shabbat.
The language of SA OC 299:6 is quite clear
שכח ולא הבדיל במוצאי שבת מבדיל עד סוף יום ג'
One who forgets and
didn't say Havdala on motzei Shabbat says Havdala until the end of
Outside of voluntary fasts1, there is no allowance to delay voluntarily for convenience.
As such, one who forgot would likely have eaten as normal.
1. Thanks to @DoubleAA for reminding us of this. I quote: "If one is fasting starting on Sunday less than 72 hours one can delay on purpose and just say the candle blessing on sat night. (If fasting more than 72 hours you'd say Havdala on Shabbat afternoon after Plag.)"