which movements are solely in-house creations

I am wondering which watches have in-house movements, and if any of the
brought in and then IWC-modified movements have the Pellatan? system?

That is not such a clear cut question to ask. Most modern IWC's have in house
movements but not all. Depends on the model and year of production.

IWC have created new In-House Chronograph and 3 Hand movements which will
debut with the new Spitfire line of watches

No brought in movements were modified to add Pellaton winding. This mechanism
is reserved for the Portugieser 7 Days and Big Pilot Movements.

For being sure about a watch's movement, you have to do research.

But as a general rule I would say, that watches with see-through glass back
have an IWC-manufactured movement.

But there are also many watches with solid case back which have in-house
movements, especially in the Pilots, Ingenieur and Aquatimer-Family. For
reasons of magnetisum and water protection these family's models usually don't
have a glass back.

I think the easiest way is to go on IWC website, for example this page:
www.iwc.com/en/calibre-family/32000.html

On top you find a drop-down menu called "CALIBRE FAMILY". If you click on this
menu, you find all the in-house movement families. All movements that are NOT
on this list are sourced from third parties (ETA, Sellita, for example).

By clicking on a calibre family you can find more information. For example
that the Pellaton winding system is also found in the Cal. 80000 and 82000
families.