Controlable or Uncontrolable Delay because of delayed inbound?
 

Got an interesting question...
UA51 EWR-MAD is delayed due to inbound aircraft
Inbound aircraft is delayed due to a medical diversion from Europe
Is the delay of UA51 considered a controllable delay (aka vouchers for food, ETC as customer care comp) because they could have found another plane or uncontrollable (bupkis) because the inbound had a diversion?

Secondly, any European scholars want to comment if UA50 will be eligible for EC261 comp tomorrow if it was delayed because the inbound from EWR was delayed tonight? Right now it's only delayed 2 hours but curious what happens if it meets the 4 hour threshold.

I'd expect a medical diversion to be uncontrollable, personally.

If it meets the threshold, yes. EC.261 expects airlines to have infinite backup planes and crews. :)

since you are not departing EU, and you are not on a EU-based carrier, EC261 does not apply to UA52 (IAD-ZRH)

Correct, although to proactively provide information, Switzerland is NOT an EU member, but EC261 applies anyway by way of agreements with the EU. So if you were leaving ZRH, it would apply.

As was pointed out, I made an assumption here...

... that was not valid for UA52, which is a US departure ...

but was valid for UA50, which is return flight from MAD and which is what I assumed -- without looking -- OP was asking about. (And, after the edit, I was right. :)