From what I can see, IceWhale is now listing a “Heat Sink Module for ZimaCube Series” separately on the ZimaSpace store, so it does look like it is intended as a ZimaCube-series accessory, not only a ZimaCube 2 part. It is currently shown as a separate store item for $9.90 USD.
That said, a few of your questions probably need confirmation from IceWhale directly:
1-ZimaCube 1 compatibility
The store wording says ZimaCube Series, so it appears likely to support the original ZimaCube, but I would still like IceWhale to confirm the exact compatible models.
2-Buying it separately
Yes, it appears to be listed separately in the ZimaSpace Store.
3-Mounting directly on the Intel die
I would not recommend doing this unless IceWhale confirms it is designed for direct-die mounting. Mounting pressure, spacing, and thermal contact are important, and removing the metal plate could risk damaging the CPU or board.
4-Fan noise and temperature results
I have not seen confirmed numbers yet for full fan speed noise or normal/heavy-load temperatures. Hopefully IceWhale can provide official test results for ZimaCube 1 and ZimaCube 2.
Would be good if IceWhale can clarify whether this is a drop-in replacement for ZimaCube 1, and whether there is an installation guide showing the correct mounting method.