Update from Spa. I thought most the teams would be introducing new units across the board but it seems like Monza is where that will happen, definitely for Mercedes who have decided to use all their tokens now and get a head start on their 2016 technology (at least for the works team). The main exception to this was McLaren, who played the rules and introduced two new ICE, TC, MGU-H and MGU-K to both cars in exchange for a ridiculous grid penalty. This now gives them several units with low use to use for the rest of the season. Sauber also introduced a lot of new units and have now used 3 of each type on both cars, so they're the only team that actually followed the strategy I was expecting.
After Monza all the remaining teams that were yet to do so have used 3rd units, though a few are still using their 2nd set of electronics and batteries. Renault decided to follow what Honda did in Spa and sacrificed the race by introducing many out of allocation units to give them two new engines to use for the rest of the season, which sees them become the second manufacturer to jump up above the 100% mark. Mercedes continue to closely follow the "Even use" line, and Ferrari are not too far ahead but seem to be following a more linear trend rather than the step increases thanks to split strategies between their teams. The works Ferrari team have now used all their allocation.
I think they all use different engines for Fridays, assuming they have old engines available which have done too much mileage to be fit to race but haven't failed yet. Ferrari haven't had many failures so I assume they're just distributing their units between the races differently to Mercedes. Kind of like what you were saying but not limited to Fridays I suspect, so they're cycling through different old units and gradually building up mileage whereas Mercedes pick one unit and stick with it. This may be bull shit but it's what the data suggests to me, unless Ferrari have had failures we don't know about.
Mercedes had issues beyond just performance at Singapore as both Rosberg, after his well documented issues with the upgraded engine at Monza, and Hulkenberg were forced to use several 4th units. Amazingly, Mercedes were the only engine manufacturer to introduce any new units over the Singapore weekend - Honda, Renault and Ferrari didn't introduce a single new units of any type between them. A turning point in the championship, or at least the engine development race, perhaps?
Update after USA. The only new units anyone has left within allocation are now either batteries or electronics, all new units of any other type will incur grid penalties.McLaren managed not to introduce any new units of any type for only the 2nd race so far this season.
As far as I can tell we had a strange outbreak of reliability at Abu Dhabi and according to the FIA documents the only driver to fit a new unit this weekend was Will Stevens with some new electronics. That leaves us with this picture to end the season:
I think the problem for Renault was bad reliability in the first 5 races (up to Spain). That left them in damage limitation mode for the rest of the year, and in Monza they took the decision to take a load of grid penalties to give them enough new units to get to the end of the year.
Apart from those 5 races and Italy the trend is quite similar to Ferrari's. The reliability definitely improved but it could be that Renault weren't pushing the performance to the limit as well to avoid a Honda-esque season.