Inquiries about VCMI (?)

[Warmonger]

is the end goal to create Heroes III in a fully moddable engine?

Yes it is.

How faithful to the original game is VCMI? For example

Very faithful, for example:
bugs.vcmi.eu/view.php?id=2076
bugs.vcmi.eu/view.php?id=981
bugs.vcmi.eu/view.php?id=1916

If the VCMI team adds new features

All gameplay features are optional.

Will there be a map editor for VCMI and if so will the map editor integrate modded towns, artifacts and spells, etc. or is that just not possible?

Soon.

In general these questions were answered years ago.