L'uva but le uve; l' is only singular.
Also, uva is a mass noun, so both a singular grape and a bunch of grapes are l'uva. It works the same way in French, too. Sometimes, le uve is used by wine makers etc. to talk about types of grapes, but never as just the regular plural of grapes.
Think of it like with English "rice" : both a grain of rice and a plate of rice are the same word. Just imagine that but with grape.
No, but it's tecnically possible because the imperative and the present tenses are similar .
indicativo presente - imperativo
tu mangi - tu mangia
lui/lei mangia - lui/(lei mangi
noi mangiamo - noi mangiamo
voi mangiate - voi mangiate
loro mangiano - loro mangino
When the verbal desinence leave no doubt the personal pronouns can be omitted, to grat reason if you give an order "Mangiate l'uva!". We can put the pronoun to point out someone doing a little pose just after "Voi, mangiate l'uva!/Voi!...mangiate l'uva!". To make the difference betwenn the present and the imperative talking we do it by the tone , writing by an exclamation mark.