toilet tank puzzle

This (wall mounted) tank is leaking at the large rubber washer that should seal between the flapper hardware and the bottom of the tank.

The puzzle is how to access and replace the large rubber washer.

The pipe connecting the toilet bowl and tank is hidden by a white ceramic pipe (apparently for cosmetic purposes) - and my impression is there's a smaller pipe nested inside the outer pipe.

Because of the pipe I don't see how to access the large nuts securing the top and bottom ends of the down pipe (to the flapper hardware at the top end, and to the spud at the lower end). Channel-lock pliers aren't working there. There isn't enough space for a plumber's wrench.

Can you suggest a good way to access the nuts without smashing the ceramic pipe? Somebody got the ceramic pipe(s) on!

P.S. What is the function of the tabbed-nut between the two thicker nuts visible in the close-up photo?

I had to break the ceramic pipes surrounding the down pipe.
That was the only way I could get a wrench to any of the nuts.
I suppose that the way it was assembled originally will have to remain a mystery.

Even so, to date I have not yet succeeded in loosening the nut that is holding the bottom of the down-pipe to the spud.

Anyway, the down pipe is a coppery bronze or brass, and has cleaned up nicely, using a scraper and a steel-mesh pan scrubber.

By the way, the overflow-assembly is cast bronze; and the casting shows the year of manufacture as 1918.

Thanks for reading!

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Take photos of tank inner parts as well as the pics you have to a real plumbing supply store ( Not the big box stores). I would do a major toilet rebuild. Replace the fill valve, Brass flush valve, spud, and offset flush ell ( or maybe straight flush ell). Make it a one time repair.