I recently found your kettle's twin (26 pat. pending copper most). Here are my non-professional findings.
1. I have the same question
2. I believe plastic. Mine came with 2 plastic caps and I have no reason to think that they are anything but original.
3. No concern there. I like to use painters tape on he kettle around the vents when cleaning vents. That way the porcelain is protected as you scrub/polish the aluminum.
4. If you insert something slightly smaller than the leg (like a broom stick or socket wrench) inside of it, you can slowly roll it around like you are stirring a drink and re-shape the legs so they fit tight.
5. I'd leave it. I think you risk popping welds if you mess with it too much.
Good luck.
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