Unstamped i797 is going to expire due to 221g administrative processing

I was selected in 2017 lottory. After RFE i got 797 approval notice for Jan 2018 to Feb 2019.Apperared for interview in Apr 2018 then till now it is in 221g administrative processing.Visa not stamped and just got lottory and approval notice.

1, What my employer can do now?
2, If employer extends 797 how many times he can extend if again and again expires in administrative processing ?
3, Can i use this with other employers for cap exempt and how?