I'm plunging down a rabbit hole! What started as an attempt to convert to plain end blades I've now found myself with a half dismantled machine. I noticed that there was some side to side movement of the blade as it completed one stroke, not a great deal but something like 1 mm of sideways deflection. It's not noticeable when it's running, you have to hand crank it slowly (machine unplugged of course). Having zero experience I'm not sure whether 1 mm is terrible, indifferent or good (although I doubt that it's good). Any back/forth movement of the blade seems extremely small. I've removed the bearing that the arms pivot on. I'm fairly certain that the lower arm was a bit too loose. The draper is a little different to some of the other machines as access to the end of the arms is through access plates at the rear of the machine. Blade is tensioned at the front of the top arm. I even think it has some form of quick release. Anyway I'll buy some suitable grease, lubricate the pivot points and reassemble, see if it cures the blade deflection.
BTW I'm not struck with those barrel type blade holders that I showed in my first pic. They could be a real nuisance with the lower one dropping when doing a number of internal cuts. Perhaps if it was turned into a magnet it may work much better.