re: They seem localized to the Meridiani region.
If the spheres were Tektites, they should be concentrated in various layers as they were deposited.
You have a meteor hit, causing Tektites to form. It is a quick event that takes place in a few minutes. The layers being deposited take years.
So you would have impactor causing Tektites.
Then layering.
Then another impactor
More layering.
So you would end up with Tekites from various impactors concentrated within layers, not scattered evenly out through the rocks.
An also, Tektites would vary in size more than we have seen.
And we should see them in any sedimentary rocks at the Spirit site.