1)
What are "Lachmi v'Es Meimai"?
1.
Radak citing Targum Yonasan: My food and drink.
2.
Radak: My special bread (Tivchasi refers to other foods) and my water. Water was expensive in the Midbar.
2)
What is "Tivchasi"?
1.
Radak citing Targum Yonasan: My cooked dish and food. This is like "l'Rakachos ul'Tabachos ul'Ofos" (8:13); Tabachos prepare the cooked food.
3)
What was his primary reason to refuse?
1.
Malbim: I prepared the meal for my shearers. Should I do Chesed so others, whom I do not know, will eat, and my shearers will be hungry?! (He did not address his obligation to pay them for guarding.)