1 Я ем зелёные бананы (I eat green bananas) is there a difference to write "I am eating green bananas" + No difference
2 Я люблю есть бананы +(I like to eat bananas)
3 Да, я съели желтый банана = I've eaten the banana, it's no longer there --- Да, я ел жёлтые бананы = I've eaten yellow bananas in my life and know the taste (Yes, I ate the yellow banana)
4 Она будет есть позже + /Она поест позже No difference in this case between the aspects (She will eat later)
5 Он съел завтрак (He ate breakfast) = he's ate his breakfast, he's finished
6 Они ели яблоки и торт (The ate apples and cake) = process, they were eating apple and cake and not something else
7 Я съел борщ со сметаной (I ate borscht with sour cream) there is no borscht any more
8 Он съел яблоко со удовольствием (He ate the apple with pleasure) there is no apple

In 5, 7, 8 you can put the verb into Imperfective and that would put the stress on the process like in 6:

5. Он ел завтрак

7. Я ел борщ со сметаной

8. Он ел яблоко с удовольствием

Besides, you can change 6 into perfective to say that they've finished apples and the cake:

Они съели я блоки и торт

"со сметаной" и "с удовольствием" are both in Instrumental case

"съел" is written with a hard sign