https://www.ipm.fraunhofer.de/en/press-publications/press-releases/electrocaloric-heatpumps-ElKaWe-projekt.html

https://www.svp.de/en/a-sustainable-approach-to-energy-efficiency/

Google “elastocaloric heat pump efficiency” thats the technical term.

Some sources claim much higher efficiency. This makes sense to me since you are not limited by your coolant’s properties so much and don’t have to maintain pressure. But I can’t do in depth research for you.

You can’t. It’s a different kind of heat pump.

If it is more efficient than vacuum-compression it’s good.

Most refrigerants are extremely toxic and extreme green house gasses. But there are safer alternatives, eg. CO2.

Skill issue can easily be made positive if you provide the information to acquire the skill.

There is no one kind of oil.

Nothing on the site. Guess we know what we gotta do. (E621)