Just to say "thank you" for this thread.
Unoriginal, I think I understand pretty much everything that you're saying (and it checks with my limited visual impressions) but would you mind defining "7" and "9" technique?
The technique system is a way to describe how a defensive lineman positions himself relative to the offensive line before the snap. There are other systems, such as the "shade" system, that describe the same thing. The gap system describes line alignments relative to gaps between offensive linemen, not on the linemen themselves.
The tech system varies highly from defensive system to defensive system. The general rules of thumb are that even numbers usually, but not always, describe head-up alignments, thus:
0 tech = head-up on center
2 tech = head-up on guard
6 tech = head-up on tackle
8 tech = head-up on end
The common ones that tend to keep the same meaning from system to system:
1 tech = outside shoulder of center, or in the A gap
3 tech = outside shoulder of guard
7 tech = outside shoulder of tackle
9 tech = outside shoulder of end
4 and 5 tech tend to change the most between systems, depending on the base front (4-3, 3-4, 3-3, 5-2 etc.) and where the coaching staff feels the need to describe alignments most. Consequently they are often out of order numerically with the other techniques. They often are
4 tech = inside shoulder of guard
5 tech = inside shoulder of tackle
Some teams might switch 4 and 6 tech, or 5 and 6 tech, etc. But the low and highs are fairly consistent. The final exception is some teams will replace 0 tech (head-up on center) with 2 tech and number head-up alignments accordingly.
Long post explaining that 7 and 9 tech almost always mean "outside shoulder of tackle" and "outside shoulder of end." Teams that line up in these techs on both sides of the line are usually trying to spread the offensive line out and have their linebackers aggressively fill and run interior blitzes.
Note that a defensive end can still line up in 9 tech even if there is no tight end on that side. They simply position themselves on the outside shoulder of a "phantom" tight end. Speed rushers will often do this on passing downs to try to get an advantage on turning the corner on slower offensive tackles. OTs will often false start in this situation because they'll be trying to time their own snap count so they can get back into their pass drops.