Problem: Take a Program that for 10 years people have been trying to stop from going on standby :-) and get it to allow standby. ok, some thoughts.
Batch The exact operation that you desire. Your already starting the powerpoint using CMD functions, so set a "Timeout" function in the batch, then set a "TaskEnd" function to close the program (or only have one big non looping presentation). Then run a "Standby" command right after that. . . Knowing the exact operating system is important to making a batch with the least additional tools needed.
Other ideas that probably would not be suitable.
Stop powerpoint and allow standby:
Kick the powerpoint off after some time, say after it has run for 5-10 minutes, then allow normal standby to occur. Problem, after the power point finnaly stops the system will still delay until it goes into standby based on the timings for power.
Simple way to do the above, do not set the presenation to loop infinate, instead have a few loops of the presentation IN the one file, so it runs a few times and then stops itself.
Start it using Task Scheduler: Set the Program to start by using the Windows Task Scheduler and Idle trigger, and have it be told to end using the "stop program if still running time" in task scheduler. Problem same as above , where is the instant standby.
Run a Program as a Screensaver there are utility programs that will run any program "as a screensaver" Assumption, if it is run as a screensaver it is possible that it would act LIKE a screensaver, and eventually close down and allow standby. Problem, most of them will activate a program, but cannot control what the program does. it is possible one might work, but I doubt it.