b. From doctrinal to Scriptural usage: a leap of faith
So why does the doctrinal language attempt this language-leap? Why not just come up with their own jargon?
Well they do come up with a lot of jargon actually, as some of the previous paragraph should show. But not all. The leap in tense of beget, which is a scriptural word, is to do with this development of son of god to God the Son, son of God the Father (Nicea), while hanging on to the authority of scripture's key idea of begetting - although I might one day have more experience to debate the successfulness of that "hanging on" (not yet). Begetting usually means causing, and is at least close in meaning. But it is very difficult to hold out that both Jesus is eternal and that in the past at some point he was caused, was begotten, etc., even if that is what the Bible says (another less-known one, although this is more anecdotal really, is that Jesus is always said to have loved us, i.e. love through action at the cross). The way around this of course is to choose a "more suitable" tense that connects us better - although in good and right recognition of God's ineffability, not perfectly - to the deeper connections within the "Godhead" that unlock us slightly from time. Assumption: the Scriptures are perfect, holy, infallible but a bit too time-locked.
A trinitarian conclusion: perhaps we can humanly make progress to comprehending a triune God's timeless begetting, being begotten, sending and proceeding, when we think in terms of non-specific effect. So in this line of thinking, we might just possibly be able to conceive of the effect of fathership and sonship... (and spiritship!) to be mutually, permanently and effectively caused. "Sustained" still seems closer to me, though.