Some snapshots of TNG is not full. So how could I get the missing fields, such as Neutral Hydrogen Abundance?
For the 'mini' snapshots, as you say, some fields are not available. This data doesn't exist, so there is no way to retrieve those fields, at those specific times.
If you necessarily need Neutral Hydrogen Abundance, it exists only for 20 snapshots, so your analysis would have to be restricted to those 20 time points.
Is there a quick way to get the smoothing length?
Which field exactly of which particle type?
SubfindHsml can be recomputed in mini-snapshots by searching for neighbors in particle data, as per its description.