I will suggest you to board from CSMT only. Because in between till kasara in most cases TTE does the ticket checking and if your coach gets checked in between CSMT and kasara then it maybe TTE will allot ur seat to another RAC passengers. Also officially Kasara is not a passenger stoppage , it is a Technical halt of HWH-CSMT Duronto so you cannot board and RPF can stop you from boarding also. So, better you catch Duronto from CSMT only in my opinion. Other railfans of that area will give you more suggestions on this.