IMPROVED in v5.972!
The actions now seem to work perfectly, except under one condition:
If timebase is beats for everything, and if this action changes the number of beats in the time selection, tempo/timesigs respond differently than items/markers/regions, causing misalignment:
Before:
After changing the first measure to 2/4, timebase = beats for everything, showing misalignment:
When timebase = time, everything stays aligned: