Escalade Blues Oil Pressure Escaping!

2005 Escalade 6.0 LQ9 Engine Low Oil Pressure when warm and at idle
(550 RPM)

170,000 miles and its been a battle for this guy..working under vehicle on
jack stands... I believe I have covered everything that I can do before going
into the bearings....

Low Pressure alarm at idle and warm.  Drained sump, changed filter added
heavier weight oil (10W40) and lucas. No good. Opened and inspected pan,
some some metal and suction pipe strainer for oil pump some debris. Tried
again no luck... Cleaned MAS sensor and throttle body to try get  idle up (all I
need is 50-100 rpm). At 1500 rpm got nearly 20 psi, engine follows rule of
thump 10 psi per 1000 rpm. Drop pan again to change out oil pump, new
Mellin pump higher volume M295 for high mileage cars. Added exact amount
of oil and again no luck.

Checked and swapped out send and have same readings... this engine I
believe doesn't have filter under it. As I attempted a manual gage, line
slipped off while running and I had an oil stream about 8 inches tall for a
moment so I think the line is clear

Engine has good power and no noise... (only once did I hear a tick tick tick
upper engine for a few minutes which cleared up)...  No oil leaking on
engine...  

So now the questions arethe following. Could I get away with swapping main
piston bearings while the engine is in place and not worrying about the front
and rear mains? I am also NOT burning / consuming oil so that indicates to
me my valves / guides are good...

Love to hear your professional opinions! Really need to fix this so I can get
some money for a trade in!

Regards

Keith