The regulations would not prohibit the running of the extra used engines from a single event in future races but they would incur further grid penalties if they did use anything but the last to be introduced engine. The teams surely would of course elect to use a completely new unit rather than a used one due to the subsequent penalty.
What the regulation is unclear about is whether a team that has used five units would still be able to use any of the five units from their "pool" or whether the FIA would require them to "hand back" one of the older units to maintain a pool of only three ICE,Turbo etc...
The regulation clearly states further use of multiple units ( edit... over their allowance) introduced over a single event are subject to further penalties but does not specifically disbar the further use of units introduced over different events. Tactically teams having a couple of disastrous weekends, maybe because a slight accident in P3 prevents them running in qualifying could introduce a new engine for the race and would then be able to use that extra engine, penalty free, in their pool for the rest of the season.
Sent from my Commodore PET in 1978