1- with this script is an incremental routing, with "refinePlace" you keep the existing routing.
2- not mandatory as you could see, in the script example, ecoRoute command also exist.
3- ecoDesign is when you provide the modified netlist, in the example, the cells modification are reduced and only to fix hold and SI/noise limitations, so around 100 cells upsizing/addition, the list is created by Primetime eco commands, to fix the timing in Primetime, our reference, and feed-backed to Encounter to place/route them.
4- "ecoAddReapeater" is where you add the upsize/add cell commands.