Just send them the pic and say you discarded it for safety reasons. Maybe take another pic with something identifying like the original receipt or something unique to the purchase and a date for proof. Make sure to get a clearer pic of the bar codes on the top as well.
Knowingly mailing a swelling battery is a safety hazard and there should not be any issue with disposing of it as long as you have something to show it was the one in that particular laptop.
If you can be certain this is not it’s normal shape
Any battery that is not perfectly flat or round is not in its normal shape. They don’t make wavy batteries.
You should add doing the salt bath outdoors to the post and recommend leaving it there for a couple of weeks to ensure it is fully drained.
I am so glad that Microsoft always tells the truth so we can just take them at their word. It would be totally different if they had a history of lying and doing shady stuff.
The downvotes are most likely because you said to go ‘against what was said’ instead of adding to it with the long term solution. It read as if you disagreed with taking it outside where it would be immediately safer.