Good questions. I think that using the exercise of writing several Rules (or rewriting significant parts of Rules, depending on the situation) over a period of several years can work very well for you or anyone whether or not they are seeking to become a diocesan (solitary canonical) hermit. One of the things that is true about writing is it allows us not only to express what we know but also to learn as we write or to bring to conscious awareness insights which were unconscious and largely hidden from us. Struggling to find the right words in a particular context often allows us to make connections with other parts of our writing (and life) we might otherwise not have made. I believe writing a Rule is one of the most important formative experiences a hermit can have. One consolidates what one has learned about living in the silence of solitude and what one has learned about the silence of solitude as a goal of human being. One learns as one writes about who one is with and in terms of God, how God has worked in one's life, how God is present in solitude, what kind of prayer one's relationship with God necessitates, when and how one does this (or allows God to do this!), what work one needs to do, what study, lectio, recreation, rest, and community one must have to be a healthy hermit, and so forth. Until one puts these kinds of things in words, until one articulates how one lives and will protect or foster them, one is not fully aware of them -- or of oneself.
At the same time one will discover where one has no words at all for something which is central to eremitical life. Sometimes we assume we are living all the elements of canon 603 until we attempt to say how and why we live them. Then we may find we have not given sufficient time or thought to a given element. For instance, in my first Rule I completely omitted "stricter separation from the world" because I was really unsure of the validity of the term. I didn't fully understand it and I wasn't sure I could commit to living what I thought it meant! It took me some years to work through the meaning of this term and to formulate what I understood in a way I could commit canonically (publicly) to. Thus, I kept it in mind, read about it, prayed and thought about it, studied the way it might fit in terms of the Gospel and my own vocation and well-being, and generally worked out what I was committing myself to (and what I was NOT committing myself to since there are serious misunderstandings of this term associated with some expressions of monastic life). The difference between silence and solitude and "the silence of solitude" was also something it took me time to understand. Writing allows for consolidation of all of this while planning to eventually write allows one to ponder and pray over what one is being asked to write and commit oneself to. This is helpful not only in formation but discernment as well.
I believe it is imperative to use the elements of canon 603 to guide one's writing and one's life as a hermit even if one is going to be a lay or non-canonical hermit. These elements define the very nature of eremitical life as it is recognized in the Church. Of course if one desires to be professed by the Church and enter the consecrated state of life one clearly has to be living these elements; they are normative for the diocesan hermit. I think, though, that, with the exception of the vows of religious obedience and chastity in celibacy, they are important for the lay hermit living eremitical life in the Catholic Church as well. In all of this they are important points for reflection, study, discussion with others, and for prayer. Your director should be able to assist you in learning about each element and in building them into your life in a way which, more and more, defines that life as eremitical. Journaling or keeping a notebook about them will also be of assistance and help prepare you for writing your own Rule of life and one that is adequate for guiding you for years to come.

You may not be including all of these elements at present and that's okay. Again, this first document is not so much a Rule as it is a summary of what you are currently living. Start then with this summary; use it as a guide for an eventual Rule and then, with the help of your director, discuss and build in the missing elements over time. At the same time and with an eye on canon 603, be aware of what you are not prepared to write about at all or what does not seem to be working well for you. Discuss these elements (e.g., not enough lectio or study time, silence is problematical somehow, dealing with friends and family, relating with your parish community, supporting yourself from work undertaken within a hermitage, etc) with your director and make adjustments with her assistance. (You can also contact hermits and ask for their input on various topics.) Make notes from time to time on the elements you have not yet included or are unready to write about, read about them, discuss them with any others you might know who do live them (other Religious and even other hermits).
At the end of a year or two during which you will have made changes with the aid of your director, consider whether you are ready to write a Rule which you could commit to live for a year or so. If you are working with a diocese and moving toward public profession this Rule should largely reflect the central elements of canon 603; again, have a discussion with the Vicar for Religious of what you are doing and why; writing in this way can provide your diocese with a significant and substantial means to discern your vocation with you. The Rule you write at this point would be your first draft of an actual Rule, so to speak, and it would be your first attempt to write a plan of eremitical living which meets your own needs and gifts. As you attempt to live it, again attend to what works and what does not, what changing circumstances need to be accommodated in some way, and what dimensions of canon 603 still seem to be beyond your vision of the life; with your director's aid make what changes are necessary. At the end of the year evaluate 1) how well your life and Rule reflect the vision of eremitical life made normative in c 603, and 2) how ready are you to make vows of the evangelical counsels? Usually you will need at least another year or two to learn about and begin to consciously live the evangelical counsels. At the same time you may need to make more changes in your life to accommodate additional elements of c 603.

Please check other posts on this idea of using the writing of a Rule as a formative process. They may be clearer in some ways or add points I have omitted here. Regarding your last two questions, if you write a Rule which is 1) livable, 2) adequate in terms of experience and knowledge of c 603 and solitary eremitical life, and 3) clearly reflects who you are, your commitment to the Gospel, and how you live that out, there is no reason for a diocese to withhold permission for profession or make you write another Rule at this point. However, yes, you need to work at it until your Rule meets these requirements until and unless the diocese indicates they do not believe there is sufficient reason for you to continue in this way. If your diocese ultimately refuses to profess you at some point along the way, the process of formation which writing this Rule has helped shape (and which has helped shape you!!) will still stand you in good stead. I don't think you will fail to realize and appreciate that. In that case you will be able to live your commitment as a lay hermit with or without private vows and with a Rule which can guide your vocation well even if you need to amend or redact it every few years or so. (Occasional amendment or redaction is ordinarily necessary for diocesan hermits as well. I have rewritten my own Rule once since perpetual profession in 2007. I may do so again this year because of significant changes in the way I serve my parish, and the place of increased personal growth work, for instance.)
I wish you well in your project. Feel free to write again if I can be of assistance.