David Lay

Should be OK for a good length of time. It's not like polystyrene or polyisocyanurate plastic foams. It's also relatively inert with no leaching solvents or evaporating plasticizers.

What make/age of concertina?

If I only had 5mm panels, I would expect to laminate pieces together to get the thickness needed for some components. Walnut is quite dense, which will affect how you attach lever posts and otherwise detail the project. I wonder, though, how using only a dense hardwood will affect the sound.

Has anyone got an active link to share for Sandylaneman?

This is what I used but at 1/8" (3mm) nominal thickness. The 1/8" material appears to be unavailable from this seller. I had another's product run out and switched makers to find that the definition of what you get as neoprene varies. I have had best success with the kind that is more firm than "squishy". ("Foam" vs. "sponge" in the Amazon descriptions, perhaps??) https://www.amazon.com/gp/aw/d/B07WVQF1DF?psc=1&ref=ppx_pop_mob_b_asin_title I put the sticky side of the neoprene to the velvet because it can be tricky controlling the amount of spray adhesive applied. If you use too much, it soaks through ruining the facing. The peel-and-stick surface is better for this, so I use spray adhesive to bond the wrapped edges. Note that in my experience, rubber adhesives eventually weaken; however, my oldest case is 4 years old and still good.

I have used velvet fabric laminated to 1/8" neoprene peel-and-stick sheets. The velvet is all synthetic - probably polyester. I have been using spray-glue to hold folded-over edges and then hot glue to hold the pieces in place. Corner blocks are wood with neoprene on the sides that contact the instrument. I like Alex's suede choice - should last forever!

Shoe polish for the discoloration! (I use a soft polyester buffing towel on my knee. I simply thought it would be a good idea. They are sold in automobile part stores/ departments.)

?? I understand the third C#''. --Still not sure a third D' is an enhancement. I think I would appreciate a D an octave lower in place of the deleted F. An F# an octave lower would also be interesting to try.

A third d' button rather than an octave lower?

Morse has stopped making instruments just this year. AC Norman: http://www.acnorman.co.uk/

Here is Bob Tedrow's short lesson. https://www.facebook.com/Robert.Earl.Tedrow/videos/592850942546296/?idorvanity=2240273277

Different makers take different approaches, of course. The Button Box let me know that they had others make their end grilles. Other components like bellows might also be contracted to others. Reeds are more often purchased from a factory to keep the amount of hand-work down. If a maker works at it, making or having others make each component in batches, then the time required is really one of component assembly. (Just like Eli Whitney's muskets!) You can generally tell by the price, the wait time, and by how much customization is offered which makers do all the work in-house.

I sought a lower tuned instrument a few years back. Used would be a great find; maybe you will still find one. New seemed more likely to be successful to me. I found only two makers with endurable wait lists. Morse has recently stopped production on their minus-1 octave C/G "baritone", however. (It is confusing to me what defines a tenor, baritone or bass instrument. Maybe a tenor is a G/D?) The other maker was A.C. Norman. He makes a minus 1 octave C/G "baritone" and a minus-1 octave G/D "bass". (Minus 1-1/2 octaves from a typical C/G.)

That would be 3.4%. I suspect the importer added some processing fees as UPS did for me to bring your total up from 2.7%. Mine had $40 added.

The dollar is high and so advantageous for US buyers though it appears to be trending down from a few weeks ago.