[DragonicRek]

if someone else signs for the item accidentally does that make the "never arrived method" impossible to pull off?

Not Impossible, but very difficult due to the fact It was signed and accepted on receipt at the nominated address.

In this case would it be easier to use the leaking battery excuse? (I couldn’t find a carrier who would transport leaking batteries, only a courier). Would it be possible to say that on discovery of the battery that had leaked everywhere I disposed of the package?

Yes.
USPS and UPS do not accept leaking batteries among other items such as toxins and weapons
If you tell the company that the battery is leaking, you will not be able to send it to them. However, they might ask for photographic evidence. If that is the case, corrupt the images