To my mind, it is unimagineable that multiple people reported unusual vibrations and it would seem a cause was not determined yet they continued to fly the aircraft. The investigation will of course find if this is the case or not. To suggest that it was a company with a good record, that they did a ground run with their balancer and found no vibration seems to belie any good practices that may have happened here. If, it turns out that they performed in flight track and balance, well, that mitigates my issue with this but even if so, a vibration with no obvious cause is just the kind of thing that pops up when people talk about how important safety is.
In 35 years of working on helicopters, of all sorts, I have seen several instances where the time was spent, the money was spent and the company bore the brunt of inconvenience to figure out wtf was going on when a pilot comment might have been as vague as " it just feels different". On some occasions it was something innocous like a loose piece of equipment ( not that that would always be innocous) but on several other occasions it was a hair raising discovery, resulting in copious amounts of beer and pats on the back after it was discovered and fixed.
Needles to say ( although apparently not really), it appears that something other than an assumed out of track or balance t/r was at work here but the assumption may have been made akin to " well, it can't be anything else and running on the ground is the same as running in flight" (not). Sometimes it takes an adamant pilot willing to risk their employment but wishing to fly another day, a mechanic who has time on their hands and likes a puzzle or an owner/operator who "gets it". Whatever it takes, doing the right thing is better than pieces flying around and the misery of an accident. This seems like one of those accidents that will ultimately have a well defined cause and explanation and I hope I am wrong about what "may" have happened here.