Partial answer to
What if the LOX/LH2 tanks were located on the Orbiter instead of as a separate module?
The original concept of the fully reusable shuttle system included an Orbiter with internal tankage such as you suggest.
However, there was also a separate, much larger, reusable Booster (also with internal tankage) which carried the majority of the ...
I found some experimentally predicted (not in-flight measured) fairing skin temperature plots for an Atlas/Centaur mission.
See the paper for details, but the peak outer skin temperature was around 600-700 K.
Source: Centaur/Surveyor nose fairing aerodynamic heating investigation