A question about non-tethered EVA risk.

It has been stated that there are no guidelines or plans with respect
to an 'over-the-side' or non-tethered EVA to inspect the RCC leading
edge.

But, given the ability of the Shuttle to very accurately manouvre
itself, is an unthethered EVA really that much more dangerous than a
regular EVA as it would seem?

If an untethered astronaut were in the payload bay, could not the
Shuttle handle navigating downwards, then backwards, then leftwards,
then upwards, allowing close observation? And then performing these
navigations in reverse, having the astronaut come back in contact with
the payload bay?

The concept of untethered EVA sound distressingly risky, but unless
the Shuttle suffered a navigational thruster failure, it would
essentially be no more complicated than an ISS docking, would it not?

Thanks.