The Power of Isengard

[Tuor of Gondolin]

The situation might have been even more complicated related to the Ring.
Discussing Frodo taking the Ring, in Letters #246 JRRT observes:

Would they (the nazgul) have been immune from its power if he claimed it as an instrument of command and domination? Not wholly. I do not think they could have attacked him with violence, nor laid hold upon him or taken him captive; they would have obeyed or feigned to obey any minor commands of his that did not interfere with their errand-laid upon them by Sauron, who still through their nine rings (which he held) had primary control of their wills.

But as to who would control Sauron's other forces, Saruman with the Ring or
Sauron with close proximity and the nazgul????? And of course, JRRT's above
musing was in Sep., 1963, and therefore probably not quite canon.

Oh, and how about this for tactical speculation: either the nazgul on their
pteradactyls or Dunedain with eagles pull off a commando raid landing on top
of Orthanc (like the German army in 1940 at Fort Eben Emael in Belgium or Otto
Skorzeny rescuing Mussolini in 1944). Orthanc might have weak spots like the
gate of Minas Tirith on its roof or balcony windows.