If you are planning to change the date of the IST-DUB/ARN leg, why don't you purchase it as a multi-city with a return. Most TK return fares include free stopovers at IST where you are not limited to a maximum 24 hour stopover in IST. With a 21-22 hour transfer time, it would be up to the outsourced check-in agent at PTY to decide if they would short-check your bags or not. If you decide to change the date of the IST-DUB/ARN leg after arriving at IST on a one-way ticket, you would be asked to pay the $300 stopover fee which is the case for ex-PTY one-way tickets before changing dates.

I haven't heard any skiplagging experiences and Miles&Smiles cancelling accounts but it would be better to stay safe.
Also with a 21-22 hour transfer time on a one-way ticket, you might be denied boarding if you don't hold the appropriate documents to enter your final destination marked on the ticket ( eg. Schengen visa, etc... ).