having tutes before lectures wont be a problem, as mentioned tutes are a week behind lectures (ie tute on lectures week 2 ocur in week 3).
i would put average attendance at tutes and lectures at about 50% of enrolment. quite commonly its even less than that. how well you can cram for exams can really save your butt. theres some indication of how this works at
http://www.maths.mq.edu.au/~rody/math237/ at the start of the semester i reckon about 250 students were enrolled in that unit, though that was down to 207. nearly 10% of students didnt bother doing the basic, compulsory work. in another subject i did (comp224 - notoriously boring), over 50 students didnt bother doing the compulsory work, the mind boggles as these subjects would be on their hecs bill.
i wouldnt worry too much about this stuff.