Hi Flocan,
Its ok to have some stamps missing eventhough you have rightly declared them in your application. I had once a conversation with Canadaian border guard while crossing into Surrey via peace arch crossings and told him about how the Canadian prs are not stamped while returning back home. He did agree with my worry and said he didnt have stamp with him and if I did want to get one would need to go inside the office. Regarding interview, you will be suprised that if they have called you for the test they have gone through your file and background thoroughly. My experience is that officers are very reasonable and if you dont have any major red flags in your application missing stamps should not be a problem.
I hope this helps.