have new hardwood floor laid in dinning room kitchen and hall.
have ~50 spots with squeaks. Almost all are at points where boards
butt ends. Can actual feel wood move with bare feet.
Installer wants to cut boards out and glue them down but we don't have
enough spare pieces and are concerned that this will not stop any other
squeaks.

Questions:
1. will tear out require replacing subfloor?
2. most of the squeaks are in the longest part of the floor (25 ft), could not leaving enough room for expansion cause this problem?

Thanks in advance

You can fix the problem with epoxy injected into small holes drilled through the flooring just to the sub-floor. 3M makes a small epoxy delivery kit with a small drill bit and mixing tube that fits onto a gun like a caulking gun.
Clean-up with vinegar if you get any spills. Be sure to make all the places to repair with masking tape.
You won't need to use much on each joint, butt-joints are always looser than the side match. If the first go doesn't fix them all you can try again. Drill into the joint at an angle.