And at which point you can no longer log instrument time/maneuvers towards
currency because the safety pilot is now PIC. The obvious exception is if
the safety pilot happens to be a CFII.

Incorrect. The safety pilot is PIC, but this does not preclude the
pilot flying from logging instrument time:

61.57(c) (1) For the purpose of obtaining instrument experience in an
aircraft, performed and logged under actual or simulated instrument
conditions, either in flight in the appropriate category of aircraft
for the
instrument privileges sought or in a flight simulator... --
(i) At least six instrument approaches;
(ii) Holding procedures; and
(iii) Intercepting and tracking courses through the use of navigation
systems.

Note that there is no requirement to be acting as pilot in command when
the approaches are performed or logged, nor event to log the time as
PIC (although one can, as per 61.51e). Logging requirements follow:

61.51(c) Logging of pilot time. The pilot time described in this
section may be used
to: (2) Satisfy the recent flight experience requirements of this
part.
(g) Logging instrument flight time. (1) A person may log instrument
time only
for that flight time when the person operates the aircraft solely by
reference
to instruments under actual or simulated instrument flight conditions.

Note that there is no requirement that the person be instrument
current, or actually act as PIC, in order to log instrument time. One
need not even be rated in category, class, and type. A copilot flying
an approach in an airliner for which he lacks a type rating can still
use that approach for airplane currency, even though he only logs SIC
time (even in a single, since the recent experience requirements are
based on category only, not category, class, and type)

Michael