After conducting the research in its entirety, a straightforward answer can be given, using the knowledge and data collected along the way, for each of the objectives defined at the beginning of this document.
1. Identify the current state of development of the Lightning Network in relation to how ready it is for regular use (i.e. not only specialized testing) on the mainnet.
The current state is considered to be not ready for use unless the user's purpose is to test and aid the network development. Even in that case, the only apt user, at the moment, is a technical expert user who can perform all the necessary operations without the help of a graphic user interface. That's not a failure of any type by the developers of the Lightning Network, it's just a part of a healthy development timeline conducted by serious professionals. It would be irresponsible to consider it completely ready at the moment.
Nevertheless, the evolution is going on full-steam and the situation might change in a few months. Even during the period of this study, the two other main implementations of the Lightning Network, lnd from Lightning Labs and Eclair from ACINQ, have become available for the mainnet in beta (lnd / Eclair). A consistent rate of development has been perceived until now and it's expected to go on like this. It would be reckless and risky to build such a system in a rushed manner.
2. Visualize how a user interface should be built in order to accommodate the new functions of the Lightning Network and provide a good usability and experience.
The interface that was described has served its purpose of highlighting the elements and the thought process that the creative team of a wallet should have in mind. Giving visual priority to the actions that are expected to be more used, balancing the amount of information that should be displayed in order to avoid clutter, and creating constraints to improve user security are all examples of the necessary priorities that pave the way to a wallet with a good user experience. The exact structure that was proposed can be used as inspiration for other projects, as well, but it's not necessary to do so if the underlying concept is being preserved.
As it would be with any other technology, be it groundbreaking or something people are accustomed to, the design principles which should be used as guidance are the same. The ten heuristics used in this analysis, for example, can help developers think better about a product from the user perspective. Obeying the Gestalt principles for a clear and easily comprehensible layout are another important approach. And, in a more advanced stage, observing how your users react to and use your product is one of the most insightful measures to improve the UX.
3. Review usability aspects and design decisions of the current solutions of graphic user interfaces, taking into account the early stage of development they are in.
After reviewing all three wallets, the general impression is of a relatively good consideration of users needs, possibly learned from the improvement of regular Bitcoin wallets through the years.
None of them is ready for use, as it would be expected, since not even the underlying structure (the Lightning Network implementations) are. Nevertheless, both HTLC.me and Eclair are more usable during this test phase than Zap. It might be a matter of what is being prioritized in the development process, since Zap is the one with the most apparent features and complexities. While, in practice, it becomes difficult to complete any task in Zap without running into many errors (some of them unexplained). Maybe an approach similar to Eclair's, which is slowly adding functionalities but making the ones that are already there work better, would result in a more productive feedback cycle from the people who are trying out the wallet to help the development team.
Apart from that, all three wallets have improvements to be made in their interfaces and should review some design decision, all of which were pointed out in the Objective three section. But the simple fact that, at this stage of development, there are already competitive interfaces focused on how the experience of the Lightning Network user will be is a promising indicator of the general awareness about the importance of UX for pleasant, safe and private wallets.