After returning from holiday and understanding your workflow/use case, I took another look to your requirement.
So using (script based) Overlays and resolving them to the playlist is not an option.
But using FTEs like followed might work:
1) When defining a FTE (Fix Time Element) you can specify the "One-Loop" option, to ensure, that the entire script (all lines) is executed once.
In order to do so, you can click on the small 'D' in the script selection dialog:
Note, that in this case the 'Count' parameter of the FTE is ignored (and doesn't need to be set to 2, but can even be set to 1)
2) In addition make sure, that for the FTE, the option 'AsEmbeddedContainer' is NOT selected, but the 'ResetScriptPointer' is selected.
This ensures, that the individual items are resolved (and no a container is created) and that the script starts from the beginning with each execution.
Also check the 'As TimeUpdateSync' checkbox (which ensures, when using a Fixed Start Type, that the back timing is reset exactly at this time).
3) You can see the resulting playlist template here:
As you can see, the scheduled time and back timing is now fully correctly updated and calculated, even for the 90sec. placeholder!
When you would now 'auto play' this entire playlist at the scheduled time, this playlist would play exactly as configured.
Note, that the FTE would cut-off (end the previous track ahead of schedule.
For example:
The 3rd track is scheduled at 14:11:27 in the above example and has a length of 6:58 and is such too long, as the FTE would start at 14:15:00!
So it would be faded-out early and not played til its end.
But...
when you would now downmix this individual part, the downmix would of course not cut-off and end the last track early, but downmix it entirely. I.e. the 3rd track would be downmixed with its full length of 6:58.
I guess you would expect, that the downmix honors any FTEs and real-time playback back time events? But this is NOT the case, as the downmix is a helper function. I.e. ProppFrexx is a playback and broadcast automation system for live-assist, manual or full automatic operations. But ProppFrexx is not a DAW, which is able to downmix a playlist as if it would be played out live. Is this what you expect? If yes, what elements would you actually downmix?, as the :15:00 Jingle would also be needed?! But the adds are added later? I.e. a post production step is regarless needed?!
Or in other words, you would need to cut-off the last track of your parts manually in such a post production step to shrink the total length of that part.