According to my own experiences you have a 0% chance of visa. why would u go for a diploma after doing an engineering course from pakistan.
Don't worry about any fees u paid to the college just take it back and apply in a university for a masters course. then u will have 100% chances .
Don't play with ur future. If chc Islamabad refuse you once and apply gain on a university then still you will have only 20% chance.