Very interesting, would be fun to compare with the same
pipe rank using the HW LFO trem generators just for an experiment.
The Voix Humaine won't have a set of tremulant LFO waves for it, that is the reason for sampling it with the trem on. Since the Voxes are quite difficult to emulate with wet samples it was decided to simply sample it with the real tremulant on (the same was done for the Hinsz at the Bovenkerk). The user has the option to either load both tremmed and untremmed ranks of the Voix Humaine, or only one or the other if memory is an issue. Then there is the option to load all releases or just the default releases for each V.H. (no, not VanHalen!)
The tremulant stop will switch between the two ranks, however it will re-trigger the attack of the sample if you are holding a note down while activating the tremulant. Being that most normal cases of playing wouldn't call for the trem being turned on mid-note I don't think this is an issue. Hauptwerk 3 does have a new tremulant feature allowing the sample developer to make it so that while a note is played and the tremulant turned on it won't apply the new tremmed note until the next attack. However the Metz ODF does not use this method.
Also since the sampled trem is recorded as is, the normal tremulant settings (pitch, amplitude, and harmonics) are not adjustable using the voicing screens due to there not being any LFO trem waves to apply them to.
All other Récit ranks use LFO trem waves applied to the main untremmed samples and may be voiced as usual.
I hope that all makes sense to you.