Can we freely distribute customized wireshark windows installer with npcap?

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Yes, npcap has a licence restriction on redistribution. This is done to allow funding of pcap support.

Your choices seem to be:

1. Submit your dissector to the Wireshark project where it will be maintained by the project team and distributed for all to access. Note that as you are already distributing to customers, the source code of your plugin should be available to those customers under the Wireshark (GPL 2.0 or later) licence.
2. Obtain an npcap distribution licence from nmap (as detailed in the ncap licence).
3. Modify your copy of the Wireshark installer to not include npcap, and request your customers download and install their own copy of npcap (as suggested in the npcap licence).
4. Modify your copy of the Wireshark installer to install WinPcap which works on all those OS's you have listed. Note that any support and development for WinPcap ended many years ago. At some point, MS changes to Windows may stop it working.
5. Do something else with other random unsupported Windows capture libraries, e.g. Win10PCap.