I don't think you're wrong. I think the 'tongues' may well have been longer than we'd previously presumed.
Here are more from the Digital Museum. I think quite a few of these (Swedish) are just post period - 1820s-40s - but can't imagine they'd change much. I've started making ours (2/95th) along these lines.
It's not military and it's post period, but might this pattern help? From H Hadfield, "The Tailor's Preceptor", 1826, found on Google Books. But I'm afraid you may need a degree in Maths to understand the text ... In the plate you're looking at figs 4, 5, 6, 7 and 8. Lots here on how to achieve a "genteel" fit. The forwardness of the tongue is entirely agreeable to fancy, apparently, so it was clearly a matter of judgement back then too.
I recall scanning these pages of "The Tailor's Preceptor" from an original copy held in the textiles research library of the Royal Ontario Museum in Toronto. At the time I was interested in the correct cut of men's trousers, which this pattern shows nicely. Also, one of the book's plates shows an adjusting belt at the trouser's rear waist.
Regarding materials, note that the first page posted by Richard above states 3/4 yard of kerseymere only--no mention of any lining material.
A soldier should be as attached to, and careful of, his musket, as his mistress. (G.O. 31st December 1788, Bombay Army)
Hello all, thanks to Richard Warren's kindly uploaded tailors notes on construction of gaiters, I've been able to recreate the 'one piece' tongue shown. The gaiter is now complete and strapped. There's a few things to improve - Firstly I need to learn how to line the tongue, although this may not be necessary! Also the buttons come too far back at the bottom, I think, and the fit has changed somewhat with the new tongue (ie they're a little baggier). But as an modification of an existing pair, it's not too bad, and a lot better than how they looked previously.
The plan now is to buy some black broadcloth and start afresh, to see how quickly I can replicate them.