I'm wondering about something here, if you could clarify. Diversity apparently existed in the Church, though obviously not diversity to the point of a hundred different variations. Let's say that there are 4-5 variations. Do you have a problem with something choosing one of those variations, perhaps not willy nilly, but nonetheless according to personal preference? I am thinking more of visiting other parishes, not so much picking and choosing in your home/main parish, where it would seem odd if everyone did it one way and you chose to do another. But for visiting, can one choose in your opinion?
Since you asked for my opinion, I'd say choosing is acceptable within limits (e.g., choose to observe "local" practice or your own, but not some third practice). I've visited EO churches and crossed myself in either the EO or OO manner: my rule of thumb has been to stick with my practice if doing so would not be looked at negatively or lead to confusion, and otherwise to switch. When I visit RC churches, the sign is the same other than the position of the fingers, and so I will cross myself when entering and exiting out of reverence for the Cross enshrined there, but I don't recite/chant prayers with them, I don't sign myself when they do during services, etc. I used to, but I more or less spontaneously "grew away" from that years ago, so it's not really an issue for me and no one seems to be offended by it at the traditional Masses I visit (certainly it hasn't stopped them from asking me to lead the pre-Mass Rosary!).
Or do you think that St. Basil (and others?) had a specific form of crossing yourself by the mid-to-late 4th century? Or are you rather arguing that one simply can't choose, that it is too important or linked with the mind/will of the Church, and that one must be obedient to a certain form or practice?
I'm not sure what form the sign took in St Basil's day, I'd have to look through some books and see if we know that information. At the Third Hour on Fridays, we sing some hymns in honour of the Cross, and they are fairly old because "the sign" they mention is the signing of the forehead with the thumb which we find referenced in some early writings. If I had to guess right now, I would think this was St Basil's "sign".
My argument wasn't that one cannot choose at all (as I hope is now clear), but merely that the possibility of choosing and the existence of variety does not mean that this is "an individual matter" or a "silly point" of no consequence. Besides the fact that just about every variety of signing has some later "theological explanation" attached to its movements and finger posture (and thus "switching" may represent a bad theology in a particular "system"), the existence of the sign itself was used as an early proof of "unwritten tradition" that is nevertheless authoritative and "canonical" in the Church and probably also of the principle lex orandi, lex credendi