Hi Jarek,
thanks for your post.
To answer your questions,
(1) You can create and sell the engines you create.
(2) The only way I can think of that you could prevent someone from modifying your engine is by encrypting the engine (e.g., the FLL code). However, for this you would need to incorporate encryption into fuzzylite, which would be useless because the LGPL license under which fuzzylite is currently distributed forces you to release the source code. If you wanted to proceed with encryption, you would need to purchase a commercial license of fuzzylite.
Cheers.