The classic catch-all term is molecular entity, often abbreviated to just entity. There's a Wikipedia page for it, which references the IUPAC Gold Book entry. Quoting from the latter:
Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer etc., identifiable as a separately distinguishable ...
Yes, it is.
In both the cases, the number of ligands remains same, which is according to the definition in Wikipedia.
For any type of structural isomerism, the premise is that the primary valence and secondary valence (as defined by Werner) of central metal/ ion remains same for all the isomers.
In this case, the primary valence is 3 and secondary valence ...
The group -COO- is called "carboxylate" if it is included in a molecule with some organic radicals attached on its left and on its right-hand side. It is a definition, due to the fact that there is a carbon atom bound to an oxygen atom. Joined together, it makes carboxy.
This terminology is used when no simpler way of naming this structure exists.