Never, ever, suggest a punishmnet for another child. It is not your role. It is the role of the Head in line with the Behaviour and sanctions policy.
OP. You wish to seek an apology for your child being in isolation for three days. Your child is your priority. You need to establish if the school followed their policies (so read them in advance) and acted reasonably. Three days, in my view, is not reasonable.
In my experience of working for an LA in a senior role (in this field) previously, it does not take three days to interview children. Most SLT get on with it immediately for obvious reasons! The Head may wish to exclude based on evidence gathered. Far too much reliance is being placed on CCTV and the isolation room in this case. Very many schools do not rely on this as sole evidence. You need to establish why other children were not interviewed promptly. Why was your DS not believed and his version of where he was not investigated to corroborate what he said. SLT do not teach in these schools. It is their job to deal with this promptly.
It is correct that they have used isolation so that an exclusion does not show up on their statistics. Ofsted do look at exclusion statistics and have been very critical of over use at some schools. They also look at formal complaints to the school when they visit. The school must, by law, have a compaints policy. Have you seen it?
Isolation in most schools is a sanction (or punishment). Do not accept that it isnot a punishment. It deprived him of lessons, seeing freinds and doing activities or even a proper lunch. It is a punishment.
Do not be fobbed off and complain formally if they do not apologise. However do not meet the Head without being familiar with the policies surrounding behaviour and sanctions. They are vital documents.
Good luck.