Took the carbine to a Winchester-approved smith a couple of hundred miles from here.
They kept it for about 5 weeks, reporting to me that the link (actually, both the original and the 'new' one) was not the problem. They replaced the carrier, replaced the cartridge guides (gunsmith told me the guides appeared to be for .45Colt) and pronounced it cured.
Took it back to them for a promised free fix. After another 6 weeks, I got it back and it works. Action is very stiff, but operable.
I'm (somewhat) satisfied. I've bought firearms from out of state many times and this is the first time I've been burned. I am convinced, BTW, that the seller did not intend to defraud; almost certainly an honest mistake on his part.
Thanks for the advice and help. Oh, I have an extra lever link if anybody needs one