Honestly MoS I don't think it will reflect badly on ds at all - I'd be amazed if you were the only family to make a mistake like that - they're fitting you in next week which is great :). It is the kind of thing I would do so am glad someone else is disorganised too 
Canon B is lovely - one of ds's favourite teachers
Comparing ds's experience at 10+ W interview with his friends, with and without scholarships, I really don't think the choice of interviewer has any significance at all
Re Alleyn's interviews, the only other child we know irl who has a scholarship interview is also a girl so maybe the paper this year favoured girls in some way (not sure how?)