Can I intentionally carry a kli shemelachto lissur if it is connected to mutar items (i.e. a non-electric although muktzeh car key attached to a ring of door keys in a seperate entity pocket, like pants pocket, throughout shabbos, so that when I leave shul Sat. night i can drive home)?
If the keys are together with a bunch on keys, and the other keys have greater “importance” than the car keys (in practice: they should be useful on Shabbos, which makes them more important than the car keys, which can’t be used on Shabbos), it is permitted to keep the keys in one’s pocket for the duration of the Shabbos.
Because the bunch of keys is not muktzeh, there will not be an obligation of shaking it out. See the previous answer on this topic here.