Yes, any new or saved Challan can be edited or modified before it is generated (before CPIN is generated).
Yes, you can save partially filled Challan in the post-login mode. After the Challan is saved, you can access the Challan using the path - Services > Payments > My Saved Challans.
You can save a maximum of 10 Challans on the GST Portal at any point in time.
The validity period for a saved Challan is 7 days