सेब को खाइए l Its more like an advice I guess. Like a doctor say - "Eat an apple". Stress on apple. सेब खाइए l Its more like if someone asked what should I eat. You would say - "Eat an apple". without any stress. But in general most people use them interchangeably without even noticing. Another example - I work at a bank. I work for a bank. I work in a bank. In Hindi all of these translates to - में बैंक में काम करता हूँ | (Mein bank mein kaam karta hoon.)

सेब खाइए is general statement .When you say सेब को खाइए it means you are indicating particular apple.

It is same meaning

Yes, both true. 1st one emphasizing on apple. it's just saying a sentence like this i want my book i want MY book i want my BOOK I want my book i WANT my book