To reply to that part of the question, yes, yes and yes.

I used them for a claim that was initially denied by KLM (reroute by train on AMS-ZYR instead of AMS-BRU that was cancelled). I turned to AirHelp and they got the job done.

Recommendation: as stated by others, if you have strong grounds for requesting compensation (which seems to be your case), always go through the airline first.

But, if the claim is denied/there is a pushback and you don't want to argue/follow up, then they're definitely helpful. Especially since because they don't get paid if the claim fails, they will only take cases they have a strong assumption they can win. They do take a sizable chunk (in my case: €420 out of a €1180 compensation).