Chances of H1B visa approval after requesting a waiver

I appeared for H1B visa interview in person in India.

During visa interview, the visa officer asked if I have overstayed in USA.
I mentioned that I didn’t. Previously I was on L1B visa status. Before L1B visa expiry, my employer filed for L1B extension that prevented any unlawful presence and gave me 240 day work authorization. I used that to continue to stay and work in USA.
During this time, my H1B was selected in lottery and I departed USA to appear for H1B visa interview.

I shared a letter from my employer’s attorney describing the events of my case.
The visa officer went away for 25 minutes and asked me to wait. After coming back, he mentioned that he understands my case, and that he is on my side. But he cannot directly issue visa because travel records say that I overstayed, and that I was previously denied visa in a different country because of same reason 2 weeks ago. (When I previously appeared for H1B visa interview before this one, I had no idea that I would run into overstay complication and I did not have sufficient documents to prove I did not overstay).
The visa officer then mentioned that he is putting my case in admin processing and requesting a waiver to get the records fixed with CBP. If that waiver is approved, they will request my passport for issuing visa.

Some more background -
The duration of my stay in USA after last L1B expiry status date to the time I departed usa was more than 180 days but less than 1 year.
I previously attended the H1B visa interview at US consulate in a Canada where I was told about the overstay being flagged on my application. I did not have relevant documents and any supporting letter from attorney. That time the case was too put on admin processing and later denied. I was communicated about this decision was email.

Can someone clarify what this waiver is?
what are the chances of the waiver getting approved?
what is the timeline I should expect for this admin processing?
what are my options if my H1B visa is denied this time too?
Is there anyone who faced similar situation or who had a waiver requested on their behalf?