I bought the OneBlade Pro at the beginning of November 2018 and use it once a week to trim my beard. Has fully met my expectations as far as trimming and cutting goes. I've dropped the thing a few times, the razor blade and trimming head have come off, but as they are very light, nothing has ever happened. I didn't find the trimming head on the Philips site, only the razor blades. But the trimming head has no mechanical function, it only limits the length of the beard.
In my opinion, the attachment for the cutting length is quite stable; I have not yet managed to accidentally change the length. However, it is not inconceivable that this could happen to you, as the rotary wheel has very little resistance.
The lengths 0.4, 0.6, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 8 and 10mm are possible. The rotary wheel engages audibly with all these settings, there is no "in between".