I wonder... Which of these terms is the most important in our spiritual development?
You mean the individual characters that make up the single term Aikido?
I would venture to suggest that, almost like our own alphabet, you cannot easily separate them and their meaning from the entire word.
Yes, you can get some meaning out of the elements.
Ai has several meanings and that can vary even more depending on which character you use to write it (and the Founder used more than one character on occasion to write it also, but that changed the meaning).
You say "love", for instance, but that is an alternate character than the one we normally use which means something closer to "harmony".
Aiki has implications and meanings that are not so simple to separate. Changing the order changes the meaning.
There is a lot that can be studied about this, but it takes a study of the Japanese language, culture, and the sub-culture of budo in Japan to get great insight.
Not being such a scholar, I've only been able to scratch the surface through the writings of people like Abbe Sensei, the Skoss', Wayne Muramoto, Dan Furuya, Draeger, Pranin, etc...
Or should we consider that all occur at once and we shouldn´t lose sleep over such things as I have done now and again?
Heck, losing sleep over these things is one way that we learn. I do think that it is a mistake to really separate the characters as anything more than an academic study.
Is one or the other more or less important, or should one be sought before the rest?
The "way of harmonizing energy"?
Or the "path of study in which we learn to harmonize with someone else's energy"?
Or "Ai = harmony or love, Ki = energy (flow?), Do = path, way (of enlightenment?), method"?
Do you see the difficulty in breaking them apart? They are related to what Aikido is, but I don't think that you can easily break them apart except to understand the terms better.