I do not think it is inappropriate to give the weight of someones opinion in line with their professional experience and training.
When you get on a plane or go in to surgery you don't expect to have others who have no experience of doing those things to have any input whatever in the decisions being made, or the actions that result.
As for justice being "seen to be done", I completely agree and often wonder why it is that of the hundreds of thousands of cases that are brought every year to our judiciary we only ever hear of those that sell newspapers and massage the basest of instincts in those who read them. Fear, jealousy, outrage, etc - all good for profit.
I've yet to see an article in the MSM that a) highlights the success of the Nordic approach (Channel 4 did an excellent piece on it a few years ago though) or b) talks of the successes (of which I know there are, or were, as my Granddad was a Magistrate before his passing many years ago).
Which again brings me to the point about the wider social context, about being informed enough to actually make a decision.
The electorate isn't - which is why we have a parliamentary democracy, we elect people who are supposed to spend the time informing themselves, listening to "experts" and thus coming to a rational and effective conclusion. Alas, even that doesn't happen - most politicians are driven by their own desire for power at any cost, rather than the greater good / good of the country, and will say and do whatever they need to in order to achieve it.
This event was a terrible one where the entire basis of judgement is being made on the final act that led to an innocent (as far as we know) person losing their life - no one cares "why" the perpetrators committed this crime, or why they thought they should do / had to, or why the owner of the vehicle valued it more than his own life.
And without that information (imo) we have absolutely no right to say we should be involved in the sentencing process, or that the sentence is too lenient or too harsh or whatever.
As a final comment, regards the Nordic system, the reason it has such widespread support from victims is because by and large they are the "last" victims of that offender due to the ever dropping reoffending rates. The issue that "y/our" victims have, is that "oh they'll be out committing the same crime again in no time". An awful feeling that quite rightly yields anger and frustration - neither of which is productive or helpful for the long term benefits rehabilitation would (and does) bring.