1. what is the best option to extend my stay in saskatchewan?
Apply for SINP Existing Work Permit sub-category under Saskatchewan Experience Category.
2. My employer is willing to apply for job approval letter, can I apply for SINP asap with this letter or do I have to show a min 6 months experience?
You need to fulfill ALL the requirements under a SINP category that you apply for, job approval letter is one of the requirements.
3. what is the benefit of job approval letter?
Again, it is one of the requirements to show SINP that your job is valid
4. can I apply for bridging work permit on the basis of job approval letter?
Job Approval Letter is only for SINP. To apply for bridging work permit, you need to go to CIC website and check its requirements. Again, apply under SINP Existing Work Permit category i think is the best and fastest option for you.
5. lets say I get SINP, can I use this for Express entry?
No you can't. Any Nomination Certificate you get from SINP cannot used to applied for Express Entry, unless you apply for SINP Express Entry category, but this one is not available yet.
Hope my answers can help.