Is there any freely available English translation of the Peṭakopadesa?
The Pali Text Society is about the best authority for English translations, and they did produce a reference text in 1977. There is not a lot of online translations, but here is a link to the digital version of the book:

http://lirs.ru/lib/The_Guide,Nettippakarana,Nanamoli,1977.pdf