There are, I think, times when 'external forces' need to be dealt with freeing a person to their own will, such as the
Gandalf/
Sauron battle at
Amon Hen on
Frodo's' behalf.
There are other times when
Eru uses existing thoughts, feelings, and even circumstantial
events to motivate the 'preferred decision' of a person. This is called at times 'intervention', and at other times, 'providence' depending on Eru's chosen method.
THEN there are times when the circumstance and decision are our own.
Here's my 'parallel circumstance', offered as "perhaps" the nature of Frodo's choice at the Council. I'll express it from three perspectives.
Type 1. It's time to go to Great Aunt Tilly's house for the annual Gargantua's Clothing contest, indulged in by almost all the family. (Insert any other family festivity you choose.) I absolutely hate to go to Great Aunt Tilly's house, as her daughter is intolerably self-centered, and her daughter's son, as a result, is an unsufferable brat. However, the family all participate, and ostracize the rest badly when they don't come. So, as much as I do not WILL (prefer) to go, nonetheless I CHOOSE to go, as it is 'best'. (denying self-gratification to avoid externally induced circumstances.)
Type 2. I've had a significant heart situation, and need surgery. The issue and damage are serious, and there's only about a 35% chance of success. I fear the surgery extremely, and do not will it (wish/prefer) it in any way. But I submit myself to the witch doctor nonetheless, knowing NOT to do it is a sure death sentence. (personal acceptance of personal 'best choice' in spite of the facts.)
Type 3. The country is at war. Troops are being slain at an unprecedented rate. Yet if I do not join and help in the effort, the whole country and all its people will almost certainly be subjugated to a new ruling entity and tyranny of an unimaginable sort. Therefore, I choose to join and help, though I do not WILL (prefer, wish) to do so. (Personal sacrifice for the benefit of others.)
The above 3 examples present potential circumstances 'external' to any (apparent) forces outside ourselves, but demonstrate we often need to and do make choices 'against our own will'' and even as we speak our 'acceptance' of the 'choice', it is as though we are hearing someone else actually saying it. It's not what we 'really' want.
Clearly Frodo's choice, if NOT the result of Providence (circumstances used in power, to motivate) or Intervention (direct causation by an external force, fits into Type 3 above.
As ALL the things mentioned above essentially LOOK the same from an external viewer's perspective, it's very hard for us to make a judgment.
Since
Tolkien did not specifically say 'an external will', but rather 'some other will', the field of options is wide open.
We COULD look at Frodo for example, see he is of split mind, and doesn't really want to go. It could well be he opened his mouth to say so. But what he spoke was from deep within him, given he knew he needed to do what was Right, rather than what he wanted.
Alternatively, we could conclude the language is specific enough to indicate an external will was indeed working through him at the time...using his own personal character and general 'goodness' as the 'hook'. Clearly, Frodo was selected far in advance for possession of
the Ring, and clearly by Providence, as that is made clear in the text elsewhere by Gandalf.
So we are not surprised when
Elrond says Frodo has been "appointed" the task.
The syntax and wording there is very clear. The 'appointment' is from an external perspective and source... Frodo just being the vessel. (The history of the ring coming to him and going from him is hugely imbedded with Providential circumstances all along the way.)
That Frodo is even at the council with the ring, all the discussion has been held, he is now fully instructed, and there is difficulty in decision... all these 'circumstances' are entirely Providential. Remember, none of the members at the Council came there for the purpose of having a Council... they showed up for other reasons and with other purposes.
The entire thing is Providence at work... but as to the moment of Frodo's decision itself... the causative element is subject to interpretation.
This is all one of the (several) highest points of Tolkien bringing the 'elements' of Religion into fantasy, 'but not explicitly'.
Tolkien was indeed, as someone has said, 'big on free will'. But he was also very fully conversant with and accepting of the reality of Providence in the movement of all things in all ways. (Which is not to say Providence is always THE moving factor... it is to say Providence is always present and at work in many ways, and we simply do not and cannot see and sort out how it all fits together.)