Herman miller shell: Need good glue remover  

After having done this...
After having done this myself, you have several options, all of which will require alot of elbow grease.
At Canadian Home Depot's, they sell an "eco friendly" paint stripper in white containers. On the back of the container it says its safe for use on fiberglass.
Apply this liberally and wait a few hours, and the left over foam and adhesive will get gooey, and it can be scraped off with the corner of an old credit card (do NOT use anything metal)
You can also use acetone to help loosen the adhesive, and scrape with a credit card.
Lastly hot water and the credit also works, or just the credit card itself.
The upholstery on my shell did not come nearly as cleanly as yours, and I spent at least 8 hours scraping all the foam off. You're in for a good amount of work to finish those chairs. Good Luck.
Oh, and when you're done, apply a coat of automotive wax to help the shine.

This stuff
was highly recommended by people trying to remove flooring mastic when restoring their MCM concrete slabs.
What can I say ? Give it a try . . .
http://www.amazon.com/Bean-e-doo-Mastic-Remover-1-Gallon/dp/B0006NL1JA

thanks for the credit card...
thanks for the credit card tip. It works pretty well for now.
Does anyone know what kind of adhesive is used to glue the padding? If I knew that at least it would be easier to find what will actually work.
As a second try I'm using WD40 + credit card

It would have been a spray on...
It would have been a spray on contact adhesive, the tacky yellow stuff used for sticking down formica, there is a clear fluid we used all the time to clean up overspray that everyone just called white oil but I googled that and its an insecticide... I think it was a mirotone product.
Or try this
http://www.adept-industrial.com.au/category.php?id=34&man_id=1&level=2
http://www.mirotone.com/NZ-Mirotone-Solvents-Thinners-and-Additives/defa...

surefire but use caution
There is a product by Goo Gone manufacturers called "Goof Off" I have used it for many things but it is caustic so you want to apply and wipe right away after scraping. I would also do an out of sight test patch to make sure it doesn't lift the pigment in the shell. It used to come in a spray but lately I've only been able to find it in the cans. This is some serious stuff as I have even removed mirror mastic with it. Good luck.

I found it 😉 I shouldn't...
I found it 😉
I shouldn't have used anything:
- no cutter blade (I actually scratched the fiberglass in a few places)
- no chemical
- no goo gone
- no credit card
I should have directly used the good old copper scourers!
It worked magic. After getting a lot done with the credit card I was up for a new idea and with the scourers I got it done in 15min for the 3 chairs!
They're now ready for a penetrol coating