Well, for starters, if Israel wasn't lying about the tunnels then the tunnels aren't "imaginary". They are real. And everyone knows that they are real:
Inside the tunnels of Gaza
With regard to your second question, that depends on the military value of whatever or whoever is in the tunnel being targeted. If you are just blowing up the tunnel to blow up the tunnel, I don't think collateral damage is warranted. If you have reason to believe that terrorists or weapons are in that particular section of the tunnel, then it is a cost-benefit analysis situation. It is unfortunate for the civilians of Gaza that Hamas has put tunnels under essentially every aspect of civilian infrastructure.