Itslimjim, anything used to improve VI is by definition a VII, whether it is a polymeric VII or an ester, Group III, PAO, or other fluid sometimes used as a base oil in another oil. An additive by definition is something added to an oil in an minority amount. Not sure that answered the first part of your first question. Finished oil's VI is a function of the interaction between the base oils' VI, any VIIs added, the viscosity itself (since it is relative), and the rest of the additive package. Some fluid additives like alkylated naphthalene and some esters have low VIs so they can lower a finished oil's VI but they may provide benefits that outweigh that negative.
Last two questions...yes and no, respectively. Viscosities at 40C and 100C are all that's needed to get VI. One caveat is with fuel dilution present, it affects the used oil's VI. Gasoline probably changes viscosity much less between those two temps, and if true, would increase the used oil's VI. Question 2: I've never seen a Group 3, 4, or 5 fluid used in motor oils that has a viscosity close enough to be used in major quantities in an oil and that also has a viscosity index over 155. There are PAOs made by XOM that are extremely viscous and have very high VIs. But low percentages have to be used due to their high viscosities and also since VI is relative, it's an apples to oranges comparison to compare it to the VI of an oil with a viscosity of motor oil.