I recently had a similar situation with my wife's Honda Odyssey keys. She's pretty rough on them. I bought cheap shell+blank eBay replacements for about a dollar or two each on eBay.

For those (and I know the BMW may be different from the Honda ones), the metal key is molded into the plastic. AND the metal parts are different up top where they interface with the plastic.

So what I had to do was to cut out (Dremel tool with cutoff and fine dental-type attachments) the new uncut metal key blank from the new key case assembly. Then I needed to fit my existing broken, separated, cut metal key into the new key case, requiring a little more cutting away of plastic material before it would fit. Transplanted internals from old to new. Then a little JB Weld and it was good as new. I even used my black Sharpie to turn the JB Weld black to match the rest of the key. Worked great for me. I expect it would work for you too.

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2001 X5 3.0i, 203k miles, AT, owned since 2014