Complete vs normal surprise was an OD&D mechanic was it not? I don't know, I haven't read those books.
Anyway, your way makes perfect sense. I don't disagree or think it's a flawed method at all. In fact, I think it's very good.
However, I will just say I don't really have a problem with a 7th level fighter turning into a wheeling whirlwind of death among a bunch of extremely surprised (I imagine the lot of them being caught with their pants down, more-or-less literally) opponents, even if it meant they were all cut down before combat even begun.
That to me is just nowhere near as problematic nor as large a stretch of believably as a non-proficient, 1st-level character firing off arrows at a rate of one-per-second, not to mention 30 of them in a row!
FWIW, it does actually specify the word, "blows" (which in my mind, do not include arrow shots) when describing the entire-round-in-one-segment mechanic on p62. That also hinted to me that maybe the interpretation that I gave above actually was what Gygax intended as opposed to me just desperately wanting the btb rules to make sense. "Melee" is also generally used to describe hand-to-hand combat, not ranged weapon fire.
... then of course we have the further confusing of the PHB stating fairly specifically that you get only 1 attack per segment of surprise. So I dunno. It's a pity those books weren't edited a bit more carefully. lol.