[Steinberger synapse 5]

Sometimes just a wee bit of oil on the screw threads can make all the difference between stupid hard to turn knobs, and ones that behave like they should.

[Missing strap button on Steinberger XP2]

Check out StewMac.  Luthier supply house.  Maybe not the official strap buttons but lots of option. 

https://www.stewmac.com

[CLH]

I did a simple Google search for "steinberger spirit hardshell case" and several options were displayed. 

[Where can I buy spare parts?]

http://www.hohner-cshop.de/en/Guitar/Headless-Bass-Series/Headless-Guitars/Hohner-G3T/Head-piece-G3T.html

[Where can I buy spare parts?]

That part is called a headpiece.  It's not plastic but rather made from an alloy metal.  Try headlessusa for a replacement.  Another alternative would be to keep this headpiece but use a string adapter... although that would require single-ball strings.

 

[Where can I buy spare parts?]

Don't know what you mean by "plastic".    Do you mean the rubber string retainer? 

[Steinberger Spirit XZ-2 manuel]

Here's a link that might help you...

http://www.steinberger.com/Steinberger-User-Manual-and-Documentation-Archive.html

[DECKADANCE 2 Not Activating]

This is the Steinberger forum which is all about headless guitars and basses.  I believe you wanted to post this in the "Stanton" forum, with "Legacy Products" sub forum.

[Synapse bass tuner hardware problem]

Don't know if this will help but here's a link to the parts page for your instrument.

http://www.steinberger.com/Synapse-Accessory-and-Part-Information.html

[Steinberger Spirit Bass / pick-ups]

Try headlessusa, they might have parts you can use.

[Spirit Truss Rod (and long-term storage)]

The truss rod on Spirit instruments is single action.  That said... what do you consider long term storage?  Weeks?  Months?  Years?  

[Where to get intonation screws for Steinberger Synapse?]

I just looked at the Steinberger website and came across this page:   

http://www.steinberger.com/Synapse-Accessory-and-Part-Information.html

 

[Steinberger Tuners]

The washer suggestion is a good one.  Also, a very very wee bit of oil (not WD) on the screw threads can help as well.  Best...

 

[Steinberger STADG06 regular string adapter]

I sell single ball string adapters on ebay under the seller id pwefx21tw.  Available in either brass or aluminum for 4 & 5 string bass and 6 string guitar.  If you're still in the market for a string adapter, my string adapters work perfectly.