First of all, the ICO is concerned with data protection which does not in itself overrule other requirements of local government legislation. As I said, where supporting papers are available, Transparency requires that those papers be published but note "where available" so no absolute legal requirement for a supporting paper to be available just that where one is, it should be published. As I've said, some councils have a co-option policy which specifically indicates that personal information about co-option candidates will be published with the agenda in which case any candidate would presumably be aware of (or be alerted to) the fact that their information is public knowledge and therefore they can chose not to put themselves forward for co-option or at least withdraw their assumed consent to this.
I should also add that the agenda/notice of meeting must be issued/published three clear days before a meeting but "three clear days" excludes the day the notice of meeting is published/posted and the day of the meeting (also excludes Sundays and the jury's out on whether Saturday can be included) so it's not the day it is received that is relevant, it is the day it is issued/posted. There is case law (can't remember the name of the case) that establishes that failure to receive a notice of meeting (e.g. when the postal system fails to deliver it, if it is posted) does not invalidate a meeting.
The Regulations you've quoted actually refer to decisions rather than to agendas which of course is issued prior to a decision being taken.
Frankly the easiest way to resolve this where co-options are concerned is to ensure a comprehensive co-option policy is adopted by the council so that everyone knows exactly where they stand.
In my council it is an agreed policy that when proposing any agenda item, councillors must prepare a paper on the subject which is published with the agenda but in reality occasionally information is lacking so that the paper is either late or tabled at the meeting. More often than not, unless the matter is of a very routine nature, a decision is deferred if members feel they've not had sufficient information or information provided on time. Ultimately, it is councillors and only councillors who make decisions.