This message was posted on behalf of Intel Corporation
As you mentioned, the Intel® Dual Band Wireless-AC 8260 is only available in M.2 form factor; regrettably, we do not support, nor recommend, using M.2 to Mini PCIe - HMC adapters like the ones you suggest. Please keep in mind that we do not recommend end users install or upgrade the various Intel® Wireless Adapter. They can only be installed by the computer manufacturer or its authorized representative.
An unsupported wireless adapter could not be recognized by the computer, it may lead to system errors or connectivity problems. Installing an incompatible adapter may even damage the adapter, the PC, or both. In some cases, this may even be illegal.
For more information about this topic, please review the following documents: