There are people more knowledgeable about Swallow's techniques than me on this forum. I'm sure if you start another thread on that topic, there'd be a lot of participation. That was a great interview. I go back and watch that from time to time.
On your valve coordination thoughts, have you ever checked out Blair Bollinger?
https://www.youtube.com/watch?v=zeC0T68GWnA
He's very into the use of valves and tuning to make music easier to play. When the valve is part of the articulation, it helps things line up. Plus, I think a lot of indy players use their instruments like dependent setups. The use of 2nd valve by itself requires a whole new set of positions, and a lot more dancing around with the fingers (and the brain). I've seen a book that requires use of 2nd valve independent (might have been Bollinger's - You've Got Two Valves - Use Them). Part of Bollinger's schtick is different tuning for the second valve. G or flat G I think.
Swallow was about getting smoother slide motion by decreasing the distance moved and direction changes. That really helps.
I've never seen a book reference trigger technique specifically, as in pushing too hard, or trying to go too fast, or jerky motion on the triggers. I can say, though, that as a not-full-time bass player, practicing double trigger technique is definitely useful and in my case needed. I had something I was playing that ran back and forth around Bb, C, Db, D (on the staff)- open, 1V, 2V, 1&2V all somewhere between 1st and 2nd position instead of using just one valve or just 1st or 1&2nd valves. There are plenty of exercises that could address that kind of thing.
Have a look at Bollinger, looking through a Swallow lens. Might get you part way to where you want to go. Might be time to write your own book, too...
