Two-fold issue really, one to do with how mantic have produced models, and the second about the list/army building.
The first depends on how the models in question were produced - hard plastic kit (like the men at arms), dedicated PVC kit (paladins with 2hw or the sisters) or individual PVC models (paladin defenders, sister scouts, panthers) - the last of these were made for Vanguard and the mega army simply includes a number of sets.
From comments made by people in the know from mantic about other models in a similar situation (huscarls from the NA range), there is no plan to release these as separate unit sets.
The second issue is that not all the units (or indeed armies) have mantic models, so the lists can’t be restricted simply to what is available. 3rd party models or conversions are often the only way to get the units you need.
There technically isn’t any difference, model-wise, between regular paladins and the paladin defenders (or the spearmen with/without veteran command).
The mega army (or trading etc) is the only way to get lots of multiples, but you could represent the paladin defender upgrade with just a couple if models for example, or add a couple of the command models to the men at arms to show the veteran upgrade.