#3 is probably the biggest obstacle. I could only find one fentanyl analog reported in the literature with methyl groups in the 2/6 positions of the piperidine ring, and that was only one methyl group. No fentanyls with 2, 3 or 4 methyl groups in the 2/6 positions could be found. The 2-methylfentanyl was reported in Patent US3923992 (http://l2.espacenet.com/dips/viewer?PN=US3923992&CY=gb&LG=en&DB=EPD)