Really terrific questions, thank you! Yes, your point is well-taken. Because many dioceses have never had the experience of discerning a vocation with a diocesan hermit (one who has lived the life for some years and actually makes it to perpetual profession), they may not know what a Rule actually is or how it works. The problem is exacerbated when the persons working with the candidate are priests or others who have never lived according to a Rule --- much less ever having written one for themselves --- and who think it can simply be a list of do's and don'ts. Similarly, such persons may not appreciate the degree of introspection, reflection, and experience required to write such a Rule. Again, when this is the case there is a much greater tendency to allow the Rule to devolve into a mere list of things one may or may not do. The problem, of course, is that such a Rule does not encourage growth or motivate adherence. Dioceses that allow the hermits they profess to write such Rules and are satisfied with them really set up both themselves and those they profess for failure.
So yes, I have to agree that this is a real problem. Canon 603 legislates a Rule written by the hermit herself, but like many terms or elements in this canon, it presumes a degree of knowledge that many diocesan officials may have no acquaintance with. When dioceses tell a candidate whom they have not worked with for any real length of time to go and write a Rule and offer no assistance, resources, contact people, or concrete suggestions or guidelines, I think there is a problem which will only become more complicated as the diocese and candidate move forward toward and with (temporary) profession. But writing a Rule is an incredibly intense and challenging piece of work (though this is accompanied by a sense of joy and freedom at many points), especially if one expects that same Rule to serve as the basis for a vocation that is canonical (ecclesial) and marked by appropriate rights, obligations, and expectations.
It is one thing to believe one is called to be a hermit, another to try living as a hermit for a few months or a couple of years and to do so successfully. But it is entirely another thing to try and synthesize what one has learned about God, oneself, silence, solitude, and eremitical life lived according to the evangelical counsels during this brief time and to create a Rule which will govern one's life for the foreseeable future for years and years!! This is especially true when that Rule needs to say essentially (and in some ways, explicitly): here is my vision of this life; here is what I am called to live and why; here is how I will embody the central elements of Canon 603, and here is why this vocation and my own living out of it is a gift to the People of God and the whole world in the 21st Century!
Let me add that dioceses and others are in the midst of a rather steep learning curve with regard to canon 603, and that some dioceses with religious in the offices overseeing the profession of c 603 hermits will do very much better in this process because they know what living according to a Rule means and requires. They may not have written one but they do have a sense of what they look like and how they function. This dimension of the diocese's own education on the implementation of c 603 is critically important for the well-being of c 603 vocations now and into the future. Meanwhile, hermits will do their best to find resources supporting their growth in this vocation. Additionally, it is likely that those who are faithful in this way will continue to redact their Rules as needed with the assistance and approval of those supervising them.