If I understand correctly you want to have one timeline column in your items that shows:
- a timeline as entered by you when the item has no subitems
- a timeline that is automatically updates from the subitems timeline columns if the item has subitems
We are close to releasing an app (Rollup Subitems) that does this. Have a good look at the screenshot and you will notice that the timeline in the main items is just a normal timeline and not one of those “funny” summary columns from the subitems.
Item 1 has 2 subitems
Item 2 has 10 subitems
Item 3 has no subitems
Let me know if that is what you are looking for