My personal take, seeing as no one asked, is that Thorn have come out very badly here.
Lifetime means lifetime, and if you put that on your blurb to sell you should make sure you back it up in a no quibble manner.
If this was me you would have received an upgraded model, fully kitted out with goodies that after all would not have cost me a fraction of their retail value.
Are they worried about having customers deliberately breaking their frames or something?
Just how often do they need to do this kind of thing?
Not great advertising at all to have issues like this popping up when one searches a brand online. Not good business at all, and as Thorn are indeed an unusual brand that is exactly what one does when researching a purchase.
Repair, once agreed, should have been done with colour and decals as required and no limitation on warranty, with a service and renewal of components thrown in to compensate for your inconvenience.
Imagine, if they had done the right thing, how someone searching for info on Thorn would react to reading about how good their customer care was? You bet, they would push the button on that Thorn they were considering with no hesitation.
Poor show Thorn, you missed a trick.
Why would they offer an unlimited warranty on the repaired frame? Their designer seems to have decided that this particular design is inherently not strong enough, so they've dropped that design in favour of the Sterling, which they think is stronger. Which they offered to him, no quibbles. He's turned his nose up at the Sterling for whatever reason (which is totally fair enough, if he's sure he knows what he wants), but you can't then expect them to give a lifetime warranty on a repaired frame that they think is inherently not strong enough and that they recommended doing something else. I think it's a bit unreasonable.