On a single TK ticket, baggage doesn’t have to originate at the first departure point. It’s totally possible to start your checked baggage at the connection point, as long as you physically present yourself at the transfer desk or check in counter in IST before the IST–MEL sector.
Since you’re doing CAI–IST carry on only, you’d just:
Arrive in IST
Go to the transfer/check in counter airside (or landside if needed)
Check the bag there for IST–MEL
Airlines care that the bag matches a boarded passenger. They don’t require that it be checked at the first station.
Being in Business with M&S Elite won’t hurt either if you run into a confused agent.
The only thing to watch is timing. Make sure your connection isn’t so tight that checking a bag in IST becomes stressful.
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