They could send up some solid fuel boosters that the shuttle could dock
with
and use for braking. How how long would it take to brake from 18k mph to
about 200mph without exceeding 2 G's?

That would be insane. You'd need "solid fuel boosters" with very nearly as
much energy as the SRB's and the ET used to *launch* the shuttle in the
first place
This is the second poster who erroneously contends that it takes as much to
brake an empty shuttle out of orbit as it did to get the fully fueled orbiter
into orbit.
C'mon guys, it only takes a fraction of the launch fuel to brake the empty
craft.
But there's no need to "send up" an SRB to brake it, just hang onto the ET
and use its spare liquid fuel for braking and then cut it loose.
^
//^\\
~~~ near space elevator ~~~~
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