1) If the new employer applies for a fresh work permit, will the old WP remain valid till the visa does not get stamped?
Yes, it is valid as long as your old "leave to remain" date is valid. You can have one work permit at one time and a corresponding leave to remain given by the home office based on the WP.
When you get your new work permit from the new employer, you will have to get a new leave to remain(finite) stamped.
2) Once the fresh WP is applied, and the candidate does not want to join, then what happens to the status of the Old Work Permit?
As per the books your immigration status becomes invalid if you are not employed for 28 days.
So, you would either
- have a different offer (means a new WP and new leave to remain) or
- find a new job within 28 days or
- go back to the old employer(sigh!) and make sure your status is unchanged by asking the employer to write to WP office and keep a copy of all this communications for your PR.
You can have multiple work permits approved at one time, but the job you accept becomes your valid WP and immigration status then on.