The tosfos shantz 25a is mechadesh that 'stira' can be achieved even without an issur yichud. For example if he goes into a private area that has a door open to reshus harabim, and also having her husband present in the city qualifies as stira but it is not yichud. Similarly, rambam writes that stira with 2 men at the same time and even with relatives that yichud is permitted with would qualify as stira.
i have a very strong proof that yichud can be accomplished without an issur yichud. According to r' yishmael that if she is found to be tahor by the water she will be zocheh to children, the gemara says in brachos31b that chana threatened hashem that if she doesn't have a child she will behave in a way where elkana will give her kinuy and she will have stira, drink the water and be found innocent to be zocheh to the bracha. ho could a tzadekes like ghana put herself in a situation where sheould violate an issur yichud. Clearly, it must be possible to accomplish stira without an issur yichud and that is what chana was threatening to do.