Both the linked iFixit guide and their other tear-down report suggest to replace the key-top
you'd reverse the process. This is why I suggest you find an authorized Apple service; or
an Apple Store with Genius bar (get appointment) if available, as their help is often free.
If no genius at official Apple Store, the other option is to have the unit inspected by the
specialist or independent authorized Apple service provider. If there is a defect, it may be
repaired under the warranty or AppleCare.
And if some tiny part that gently snaps into the hinged section from the top piece is broke
then one or the other may require a replacement. At some point, maybe entire keyboard?
• Retina MacBook 2015 Keyboard Key Butterfly Mechanism Replacement - iFixit:
https://www.ifixit.com/Guide/Retina+MacBook+2015+Keyboard+Key+Butterfly+Mechanis m+Replacement/44040
"To reassemble your device, follow these instructions in reverse order." -- see comments.
Perhaps a replacement key reseller may have some ideas about the Spacebar key parts
& how to remove-install their part successfully? This link shows just the small keys + video:
http://www.replacementlaptopkeys.com/apple-12-macbook-keyboard-keys-replacement- butterfly-2015-1/
Since the computer is relatively new, and shipped in the first quarter of 2015, less is known
outside of the official channels or through independent investigation or repair attempts.
While this ASC has been suggested to be a technical forum, there are times when it isn't.
So that's when the advice is, to try & find an Apple specialist or an official Apple genius.
Sorry to not be of much help... best of luck in this matter.
Perhaps someone else may see your thread & offer ideas...! 😎