good question. I presume extra piutim were said by those who finished early, while they are waiting for the rest – otherwise, there is no purpose. People should not leave even without piyutim, it is an aveira to leave someone daven by himself and expose him to dangerous (or improper) travel. Piyutim just help ensure compliance according to my imperfect understanding. For a similar reason, there are mirrors near elevators in high-rise buildings. Waiting for an elevator is a tircha, and people notice it less if they can spend it checking themselves in the mirror. This may be outdated in the time of cellphones.
Kiddush will be after everyone is finished, unless you are suggesting doing Kiddush while others are finishing.
Also, I think, your family will feel differntly waiting for you engaging in davening and a mitzva of securing walk for the friends, rather than in drinking and eating while they are waiting, even as you are engaged in a mitzva (that they are waiting for). Not sure about halakhic category here, but it just does not sound right.