When you had the cylinder head re-done, how MANY of the valve guides needed "work"? THAT is important in this situation!
Certainly, exhaust valves can "burn" due to combustion heat that is higher than the design standard of the valves themselves. BUT what really happens, as my machine shop operative noted once, is that as the valve guide wears, it will allow the valve head/valve itself to wobble/bounce as it seats, before the valve finally resides in its valve seat. With time, that little wobble CAN deform the valve head enough to allow for a little less "seal" as it seats, then the exhaust gasses can start seeping through, not letting all of the heat in the valve head be transferred to the seat area for cooling, and it's that combination that can lead to exhaust valve "burning".
The side loads placed on the valve stem ends were greater with the non-roller tip rocker arms we had on older engines. Getting one of those engines past 80K miles without ONE burnt valve was the exception, back then, when most engines didn't turn past 3000rpm most of their lives (in the USA).
You have an engine that is computer controlled and each cylinder is optimized for fuel/air/spark efficiencies. If all is working well at part-throttle, then WOT should be equally-well done Yet you have two adjacent cylinders with problems. To me, that kind of makes it hard to claim just "fuel" as the main issue, especially for an engine with the higher distance-use yours has.
At least in the USA, E85 from all vendors is not true E85 (unless you purchase verified racing fuel in drums). Some brands can have more than the other brands, just because the fuel spec is written that way. Key thing is that it's NOT an absolute spec, as E10 is closer to being, at least at the fuel station level. E85, E10, etc. start as regular-style gasoline and the ethanol component is "splash blended" at the fuel distribution terminal. With blender pumps, it can become more accurate, possibly, at the consumer level.
E85, due to its moisture absorption properties probably CAN end up at the bottom of the fuel tank. That "phase separation" thing it is noted for (which CAN also happen withE10!). So how much of it "phase reacts" (and how soon it happens) can depend upon ambient humidity at the time of fueling and how much humidity can creep into the fuel system vent mechanisms and such.
Alternative fuels can be good and such, BUT I suspect you'll discover that the best fuel economy is achieved with E0 fuels and optimized tuning for the lowest cost per distance unit. IF the E__ fuels do have a lower unit cost, the fact it can take more fuel/distance can make such fuels more expensive in the long run . . . unless you optimize your engine to run those fuels ALL of the time.
For those concerned about crude oil use, then we should IMMEDIATELY return to "pure gas", even leaded pure gas. It takes approximately 5% MORE crude to make unleaded fuel than it took to make the older leaded fuel we had in the earlier 1970s. THEN, we lost about 6% fuel economy with E10 fuels, with greater fuel economy losses as the ethanol content increases for engines optimized more for E10 than for E85 (i.e., design content of the engine and such). It's a better situation to design for low-ethanol use and then adapt to higher ethanol blends than to go the other way (down-convert from E100 to E10, which is much more problematic unless we might get racing gas of 100+ Research Octane readily-available again.
I fully understand the reason we went to unleaded fuel in the first place, other than purely environmental reasons. But then we had to decrease compression ratios for even greater inefficiencies, at that time, which meant greater fuel economy losses. Greater-evolved combustion dynamics engineering and electronics now allow compression ratios which were only used for pure racing engines in the later 1960s on leaded racing gas. In those same earlier times, there were a few gasoline brands, regional ones, who sold 100 Research Octane unleaded fuels with no real issues of valve seat recession or fuel system damage from "alcohol blending components". How could they do it then and we can't do it now?
CBODY67