OK, can you elaborate on the rejection letter?
Was it in fact the manufacturer?
Or was it the distributor?
Or was it the LBS?
Were you provided a verbal rejection and reason or was it detailed in a written assessment.
A reasonable LBS could make an assessment themselves and manage a frame replacement with the distributor dependent on their relationship influence etc.
However the standard process is the distributor either receives the frame or photographs and if unable to make an assessment themselves they will send the pics (usually not the frame) back to the manufacturer/mothership office of the brand.
After assessment a decision is usually made and more often than not options are provided to the customer.
Typically those options are positive in nature ranging from a replacement frame at no cost but you pay for transfer of parts in most cases, or a reduced price replacement frame dependent on shared accountability of fault to no go, but no go is rare.
Unless you have written correspondence from the actual decision maker logically detailing the reason for the failure you cannot ascertain if it has gone through the proper channels so I would chase that first.
And yes if you wish to pursue this if you have in fact received “manufacturer” decision you could visit a carbon frame expert here in Australia, pay a fee and have them make an engineering determination that would either support your case or not.
On the face of limited information I question whether this actually got to the “manufacturer” or whether it was a decision made purely at the LBS to fob you off.
Whatever you do don’t lose, be firm logical and seek clarification, provide as much detail as you can, you may find you end up meeting in the middle somewhere.
As a reference in eleventy billion years of riding I have only had a handful of warranty issues myself for my stuff that were remedied without a hitch as follows;
Rear triangle on a very expensive handmade usa duallie was welded incorrectly and off kilter – I secured a written engineers report from a well known bicycle engineer, this sped up the process and local distributor honoured replacement in a week rather than waiting for usa response
Super record lever had a part break internally – explained the situation to LBS and provided a detailed explanation that they passed to distributor, part was replaced, lever fixed and returned in a week
Flatlock stitching on an expensive pair of knicks coming undone majorly after 2 rides, ended up being a known batch problem and resolved on the spot
Schwalbe tyre issue, ended up being a failure in manufacturing process impacting those tyres world-wide, full refund as no stock or replacement available at that time
That’s about it for me, but a reasonable situation can always be resolved I reckon.
But I had previously dealt with customers/riders with exy bikes who had carbon frame failures and out of about 6 by memory each were provided with a new frame although one took 6 mths to resolve due to frame shortages
Another guy came in after he had crashed and asked if he could be helped after crashing his new pride and joy and his fault, spoke with the distributor, he wasn’t a regular customer a but a teenager who saved for ages and arranged a crash replacement at a reduced price.
Recently there have been issues with a batch of xtr pedals coming away from the spindle and the response led by Shimano is immediate exchange with replacement.
As I indicated I am surprised you say you have a no-go, but you indicate there is a he said she said between you and the LBS on incorrect seat collar adjustment and these little facts add to your dilemma on unfair wear and tear (you) versus warranty (manufacturer/distributor) versus duty of care in the build up (LBS) of your bike.
I’d go back and re-establish discussion with the manager or owner of the LBS, take notes now on what has transpired and take notes on nature of the conversation yet to come as you may need them if all goes wrong, but I’d like to think you can reach some arrangement.
Hopefully this gives you some perspective and may help you in some way.