Best conformation shot I was able to send off. Photogenic mare does not like to stand square.

Confetti’s back
The fitter noted only a bit of soreness, mainly towards the back of the saddle if I’m recalling correctly.  Her back looks generally good.  She affirmed my belief that the pony has big shoulders, no withers, and needs a relatively narrow tree but with appropriate flare.  She also confirmed that a crupper is likely always going to be necessary: it’s just how she’s built, there’s nothing to hold the saddle back.

Usual Thorowgood setup.

Thorowgood
The flocking on the right side was uneven.  While some of it might be attributed to the saddle rack (oops, my bad, I haven’t been as careful with the flocked saddle!), it looked like a bigger issue than that.  When we put it on Fetti and had her walk forwards, the saddle consistently shifted slightly to the left, consistent with the flocking being uneven on the right.  Oddly enough, this is not something I’ve noticed at all while in the saddle – but then I’m not entirely even and I compensate for that, too.
Flocking issues are fixable.  It might be worth trying a narrower gullet plate.  However, on further examination, they did not have any narrower ones with them – so we opted to ignore that option for right now. 
Eurolight
I learned things about my saddle from the fitters.  Crazy how that works out!  My Eurolight looks to be one of the first Eurolight models, with absolutely no seat padding.  The seat, however, can be shimmed to influence rider-fit.  I thought shims were only for horse-fit!  It has more of an endurance seat than a trail seat, but doesn’t completely match to the current endurance seats.  This saddle consistently shifted slightly to the lft when walking, too, but I think that was less so than the Thorowgood?  The fitter noted that we’ll want to adjust the angle on the left shoulder.
The billets and the girth didn’t fall quite in the right place; I should consider a contoured girth to help set it in the right place.  For the record: inexpensive 24″ contoured girths are tough to find, especially once you rule out any mohair/string girths and prefer to avoid leather!

winter Specialized photo

I’ve had a tough time pinning down what exactly it was that made me feel off-balanced, but once I hopped up, it was clear to all of us involved that I was not-balanced in the Eurolight as-is.  We did some shimming with my Eurolight to see if it might fix things.  I pointed out that I have a difficult time getting my thighs on the horse, and thus my toes tend to point out; this is exacerbated when I have pommel bags attached, as my knees run into the bags.  Given that, they pulled out a 16″ Eurolight demo (this is why I wanted a Specialized dealer, too!), shimmed it up, and had me give it a try.   Wow.  What a difference!  The seat didn’t make me feel trapped, my thighs went on (reasonably) well, and the seat was way more comfortable.  Hey, it’s the little things.  The demo model had a trail seat rather than the more-popular endurance seat.

My position
I absolutely loved this: not only are they saddle fitters with Specialized experience, they have a centered riding background and I got a mini-lesson to go with the saddle fitting.  I am, unfortunately, a large part of the issue here, though my saddles haven’t made it easier.
I really struggle with body awareness.  This is not new.  I know this.  I don’t know where straight is; I don’t always know quite how to do something that my brain understands but my body can’t piece together.  Mostly, I tend to arch my back too much and not lead with my seatbones or anything.  Let’s face it: I grew up riding hunters and getting the job done, but I never made it look pretty and I was never super-effective.
The lightbulb moment: use one of Fetti’s lower braids to physically pull myself forwards into a posting trot.  This motion has eluded me for years.  Using that braid connects all the dots and has entirely changed how I post.
I do a better job finding neutral/correct position at the sitting trot; if in doubt, return to sitting, then try posting again. We could have put me back in my Eurolight for proof-of-concept, but it was pretty clear mine wasn’t helping me in the battle to fix my position, so we opted to skip that.
The fitter also had me drop my stirrups a hole in the dressage saddle.  It’s been two weeks and I’m still riding with it at that length.  Combined with the new posting mechanics, it’s working fine.

End conclusions
1. Adjust flocking in Thorowgood dressage saddle to help with lateral shifting of saddle & white hairs
2. Relearn proper posting mechanics and true neutral, use braids to help acquire muscle memory
3. Acquire 16″ Eurolight with a trail seat

#1 the fitters did for me.  Yay!  #2 is my homework and I’m continuing to work on it.  #3 rapidly became challenging.  I have been casually watching for 16″ Eurolights or Internationals ever since May.  They do occasionally show up, but often at more than new-Eurolight price, and frequently with a wide tree.  Fetti does not need a wide tree.  I do not need to pay more than new.  I hadn’t even thought about what seat I needed the saddle to have, and already I knew my options were limited.  Adding the trail seat parameter took my used options down to zero.

Funny how things work: it was two years exactly from when I first set the Eurolight on Confetti to when I listed it for sale.

It’s not in me to buy new very often, but to buy used, there have to be appropriate saddles out there.  I put my 15″ Eurolight up for sale on Facebook at what I thought was a reasonable-to-high price.  The market disagreed: within 24 hours, and before it even made it up on two of the three groups I listed it to, I had a buyer at my asking price.  I sent an email off to the fitters, made my decisions, and placed an order for a new 16″ Eurolight.  There’s a 3-4 week wait time for new saddles right now, so this should get it to me before our early October ride if all goes well.

I’m happy to have answers, and I’m excited to see where this takes us.  Fewer saddle fitting headaches, yes please!