Im glad you mention this in details I really think its the final word here. No soldier would have gone through that process in the field, he would have put that energy into finding another jacket, exchanging with someone else or just wearing a short jacket until it wore out, was lost or could be replaced. Again if our OP is a taller man, I really think the short jacket would be a good compliment to his impression. I know its annoying as I say this as an Owner of a Columbus Depot jacket that I think is too short on my 6 foot frame, but I got it used and just treat it as "this is just what I was issued".
Right, exactly. I concur with your original post. While it can be inaccurate and lazy under many contexts to just say "poor fitting is accurate", this is one of the most perfect examples of where it is the reality, really. It's a government issued shell, countless examples of images of them (and likewise all manner of Union) that do not fit perfectly, and a
shell especially will be noticeable if it's a bit short.
I myself am in the 5'10-6' range, have the same issues with uniform especially with sleeves.
Moreover something generally - what he's asking is almost impossible in normal circumstances to alter; one option is more or less just turning it into what looks like a cut-off frock coat, for this alteration you would have to entirely remove the foreparts of the coat (which also means reconstructing the padstitched canvas attached to the forepart too, interlining and/or anything else - essentially re-making the whole front) and really, the back too... all the while of course, having the exact same coat fabric to do so, or something very close for it to look right at all.
This is also why, whether it is 400 years ago, 160 or today you always cut things too
big rather than too small - you can always cut down seams if their allowance is too big, re-stitch sleeves if they are too long the first time you sew them etc... but if that sleeve is too small? There is no lengthening them unless you added some (rather odd..) seams at the top for your extension, you could potentially make a false cuff and lengthen it if done very well, and most likely you'll have to make new sleeves.
I mean no disrespect to the OP but it might be a useful comparison to say, this is a bit like asking to turn
shorts into
trousers.. you can do it, but either you will have weird looking seams where your extensions are, or almost all of the main, definitive pieces will need replacing, with the same fabric.. which would be infinitely more hassle than just making or buying new trousers. Meanwhile,
trousers to shorts is far easier, almost amateur work. (Likewise would shortening the Shell's length.) With the frock coat comparison - likewise if you lengthened it, which would be easier to do as just the skirt would be replaced in most circumstances, you would still have the dodgy business of procuring as exact fabric as possible, which may not be an option. All of these examples can be done, but what the guy is asking for is more akin to restoration work put on heavily damaged original pieces in museums and collections; might as well buy a second coat.