This is very elementary. It is to be one once a year because it is based on the precedent of the passover being done once a year.
You don't need any new testament quotes to know that.
You have summed up nicely the WT's whole argument as to why it is supposed to be an annual memorial. Unfortunately, Paul and the Corinthians seemed to have failed to see it that way.
Like I mentioned to "designs," I have no problem with someone choosing an annual memorial. But the command to "keep doing this" and "as often as you [do this]" is just too ambiguous to say that an annual remembrance was meant. Moreover, the practice of the Corinthians, as described in 1 Cor 11:17-20, just doesn't agree with an annual observance.
Paul is addressing questions raised by the Corinthians, 11:17-34 being about "when you come together in a congregation." The context really wasn't about the Lord's Evening Meal per se. It was about "divisions" among them when they had their gatherings. Paul was saying that, under those 'divided' conditions, having the Lord's Evening Meal was inappropriate, since the 'bread' and 'wine' symbolized something Jesus did for all of them. Therefore, the 'divisions,' selfishness, and unconcern for one another were a contradiction to what the Lord's Evening Meal represented. But behind that discussion is the presupposition that the Lord's Evening Meal was a regular feature of "when [they] come together in a congregation."
Thanks for your comments, Rattigan.