Albert Park 5.303km / lap - Race: 58 laps = 307.574km
Vettel 153 laps = 811.359 km
Hamilton 147 laps = 779.541km
Bahrain 5.412km / lap – Race: 57 laps = 308.405km
Vettel 132 laps = 714.384 km
Hamilton 140 laps = 757.68 km
China 5.451km / lap – Race 56 laps = 305.066km
Vettel 130 laps = 708.63km
Hamilton 135 laps = 735.885km
Vettel = 2234.373km
Hamilton = 2273.106km
So in total the will do around 5500km per engine and there is certainly a safety margin too. In 2015 the Mercedes engine with the most km had almost 4900km. If you reduce the fuel flow to lmp level then power and revs would drop and the engine life would be significantly longer. Power per liter and max revs are very good indicators for engine stress. If they could run 2 liter engines they wouldn’t have more power because of the fuel flow restriction that’s true. But the average pressure in the combustion camber would be 25% lower to achieve the same power. The temperatures would be lower and the max revs.
There was a discussion if the f1 engines should be allowed in lmp1 to give the teams a possibility to sell more engines / reduces costs per engine.
In practice they have to ascertain tire and fuel usage. Therefore they need to run at race speeds. Otherwise they would gather useless data. Only the in laps are clearly slower.
Mercedes have around 1600 engineers (800 engine, 800 chassis) working on their car - Ferrari around 1200. Normal midfield teams have around 400 and Haas has 250 but the chassis is developed by Dallara and not in house.
I don’t know how many people work in a lmp1 factory team but I don’t think that they have more than 400 engineers.