I my experience Fazers don't suffer serious electrical faults, I've never replaced a CDI unit, it's always the simple/obvious. That said if the bike has been messed about with, or has had an alarm removed badly anything is possible. Either will cause serious problems and if that's the case, you're going to have to trace the loom along it's length. I've fitted an complete new loom of a few occasions due to poorly fitted/removed alarms or owners, cutting into it, or badly fitting accessories which reroute, crease, cut or damage the loom.
I know you've said you've checked X & Y but there's really not a lot that can or does go wrong with them, not with standing the above statements.
As mentioned, the ignition switches can often be an issue, these are known to fail/jam etc, as does the starter relay. They're also very sensitive to battery voltage, 0.1 of an amp will make the difference.
Look for the obvious, cut or messed about loom.
Some alarms (Datatool System 3 being the most common) aren't removed they're looped out with in it's own box. This is often poorly done, the box not being resealed and water gets in. Another is Meta, these are cut out, often badly causing damage.
Pull the starter relay and check to see if it's been opened, they corrode and owners open, clean and refit them, only for them to fail again later. Check the starter and kill switches for wear/damage/corrosion any of which can/will give you a good reading but not work in practice. Coils rarely fail, but as mentioned low circuit side can, check for broken, worn, corroded connections. Check the instrument brinicle for it being opened, they often get opened to fix the liquid crystal display, fit blue bulbs, different clock faces. Damage can occur while it's open, and/or moisture gets in.