In my experience (which has nothing to do with Windows),

Unfortunately, that renders most of what you've written entirely useless to the OP.

• There are no such things as "lightweight processes" on windows.

  There are threads, and processes. (And fibres, but they're not accessible from Perl.)

  Every process consists of at least one thread; and is started fresh; never cloned from an existing process.

• There are no such things as forked processes on windows.

  Fork is emulated using threads.

In brief, using multiple processes and multi-threaded processes sounds to me like a bad idea in general,

It's quite normal under Windows.