Very sound approach, Andy. I say that the brim binding needs replacing because I have been told by Patey and Ascot Top Hats that the old ribbon would de damaged on removal (either because it is already worn or that the removal process would weaken it to the point of disintergration) but I cannot see how one cannot preserve it if you just be very careful. I suppose I would remove the trimmings before sending it to Patey's or godknowswhere.
In any case, I'd only attempt it is I got a proper brim iron as that is small enough to reach the curves of the brim rather than a massive household iron. I have a topper that needs the merino replacing as well due to extensive moth.
The merino is wool twill/serge and I suppose it can be had from many fabric stores.
That's exactly right. On the topper where I replaced the brim binding, the areas of the ribbon that hadn't been rubbed away from 100+ years of constant wear and tear, were quite flexible and strong, and if a topper still has intact brim binding I can see no reason why it would not be salvageable. My guess is it's more trouble for them to save it, instead of just slicing it off, so they refuse in order to save themselves some time and effort, at the unfortunate cost of historical authenticity.
The inner brim/crown is a difficulty or impossible without a proper hat iron, I can't argue with you there.