Dave, you pose a few interesting and significant points well worth discussing.
"I typically seat bullets for this rifle about 20 thousandths from the lands. However, some factory loads with a similar bullet shot great from the rifle. My measurements show that the bullets were seated about 100 thousandths from the lands. ... This shows me that a deeper seating depth for my load is definitely worth trying. "
I wonder how you got to 20 thou off; was that from tests or just picked?
It seems a LOT of people have been deluded that seating at or very close to the lands is more accurate. As your test of factory ammo at five times further off shows, that 'close to the lands' idea is nonsense more often than not. In years of loading for quite a few factory rifles I'm yet to find one that shoots best at the lands, usually off from .020" to .035" but sometimes as far off as .100".
Rules of thumb are good for measuring thumbs, only personal testing can determine what a given load needs to shoot well in a given rifle.
My question is this - should I just knock the bullets in my loaded rounds back (incrementally, of course), or should I just load new rounds. I have a slight concern that knocking the bullets back on loaded rounds may break the case's neck tension and therefore have different characteristics than rounds with bullets seated initially to that depth,.."
Use what you have. Seating them deeper will have no detectable effect on "tension".
I have no X-ray vision high speed camera so I can't say for certain what happens inside a chamber those first few nanoseconds after igniton, nor can anyone else, but many of us believe the pressure expands the case firmly against the chamber before most bullets can even clear the neck! If we are correct, fretting about "bullet tension" is basically irrelivant and all that matters is the timing of the bullet hitting the lands. Thus, if the 'neck tension' is sufficent to keep bullets from falling out during handling it's enough and the point of varing OAL is to tune the entry into the lands.
" have quite a few loaded and this would be a simple way to make test rounds. What do you guys think?"
I think you are correct.
Like powder charges, best OAL (bullet jump) is a range, not a specific point, plus/minus nothing. It's easy to load on the ragged edge of a good range so small variations can kick the bullet outta the group. Work to find the full width of the good shooting range - powder or seating - and load in the middle of it so you won't get unexplainable (?) "flyiers."
I'll make a suggestion to simplify and speed your tests; at first, just to quickly eliminate any poor OAL spots, shoot TWO SHOT groups. You know how your present load and the factory stuff shoots so what you'll be looking for is a seating range that tightens it. That closing will be gradual if your tested seating steps are small.
Seat your already loaded bullets deeper in steps of perhaps 5 thou per pair. That's close enough so that as you approach a node you should see the two rounds getting closer and as you leave the node they will get wider. (NOTE: Any two rounds that are VERY CLOSE together but don't follow the OAL pattern of getting closer and then wider are probably meaningless flukes. Again, the good seating depth is NOT a specific point.) After you have identified the OAL that seems to do best do a few more test groups at and around that to confirm or disprove it's the best you can get.
Good luck, and let us know how it goes!
