1. A PG Diploma is usually a professional post graduate qualification for those who either already have a bachelors degree or equalvent or those with a masters degree, while a masters degree is for those with a bachelors degree who wants a higher qualification, be it professional or not, and this masters is usually a pre-qualification to a PHD program.
2. Yes, most Masters program will either need ILETS or TOEFL if your bachelors degree is not from an English speaking country, such as from the US or Canada or Australia or UK.
3. The requirement depends on the program and also the University, check the respective program and uni that u want to attend.
There is no such thing that even though u have a bachelors degree already and when u apply for the PG visa, it will be rejected, its never a rejection for sure. U may also get rejected if u are applying for a Masters Visa, so it is dependent on the visa office and the visa officer and also yr submission of all the required documents.