1. Without going into technical details, an applicant must:



a. Have a bona fide job offer for a qualified position from an employer and the nomination by your employer must be lodged with DIBP;



b. Your experience and qualification is recognized by Australia;



c. Have an acceptable English skill (like IELTS 5 with a minimum 4.5 in all subparts).



2. Same requirements. 457 subclass is not a prerequisite for any migrant visa by skills.



3. There are many. The most common visa bridging 457 subclass is 186 subclass.



4. See here:



https://www.border.gov.au/Trav/Visa-1



Addition:



1. 457 subclass is the most common visa subclass. Other visa subclasses with employers' nomination carry special restrictions on the employers.



2. Only 186 visa subclass. 187, another visa subclass, although requires employers' nomination, also carries special restrictions on the employers.