In my mind, it is all very simple. I'm confused as to why there is so much confusion, although the description of the experiment in the paper could use some polishing.
If you look at the photo, you will see that the two voltmeters are measuring the same thing: the voltage drop across the reactor. The resistance path through the reactor is known. Therefore power can be easily calculated.
The brown resistor is not connected to either voltmeter in the photo. It is there and available as a sanity check. It is probably connected in series with the reactor. In other words, the voltage drop across the brown resistor can be measured to determine current through the reactor.
I think it's quite clear in the report that it is the voltage across the resistor that is measured.
Page 18 in the report:
"In the left in the figure there is two voltmeters that measure the mV of the current passing through the 1 Ohm brown resistance."
But lets say the report is wrong and you are right. That the voltage measured is the voltage across the reactor. Then it's is correct to calculated the power as they have done in the report only if the resistance in the reactor is always 1 ohm. But how do we know this? Because Rossi say so? This would be as as stupid as when Rossi on previous occasions claimed that all water was vaporized without measuring it. It would have been so simple to measure both the voltage across the resistor in series and the voltage across the reactor. And with those two values it would have been simple to calculate the power in to the reactor without the need to trust any "Rossi say" about the resistance in the reactor.
It seems like we have two different possibilities. Either it is the voltage across the resistor that is measured (as it is described in the report) and then the calculation of power is wrong. Or it is the voltage across the reactor that is measured as you think. And then Rossi once again has made a stupid test set-up that has to trust "Rossi say". When it would have been very simple to also measure the voltage across the resistor and eliminate "Rossi say" from the calculations.
I said some years ago that there is only three possible alternatives:
1. The e-cat works and Rossi wants to prove that it works. But Rossi is extremely incompetent and refuse to listen to advice.
2. The e-cat works but Rossi don't want to prove this without doubt.
3. The e-cat doesn't work as Rossi claims.
I think this is still valid and my guess is still number 3.