SetCellText cannot be used, as the error message you quote clearly states.
To understand this in more depth, please search this forum for 'SetCellText' and read the numerous previous discussions on this topic.
The schedule is a view of the BIM, so the data is displays is read-only.
To modify it, you need to modify the model appropriately.
Your item 1 is a possible approach. It depends on your model, the elements, and their data.