1986-1997 Ranger, regardless of engine size, should run 30 psi fuel pressure, +/- 8psi
1998 and up Rangers used 55psi

Your fuel pressure reads OK, and Fuel Pressure Regulator(FPR) seems OK as it is set for 42psi max so 41 with vacuum line unhooked is spot on
The drop to 0psi doesn't read as OK, but wouldn't cause your lack of power if pressure remains above 20psi with engine running
0psi would mean engine might be hard to start, try cycling key on and off 3 times then try to start
Each time key is turned on the computer will run the fuel pump for 2 seconds, but only 2 seconds, that should increase pressure by 10psi each time

Normally, with engine off fuel pressure would stay above 15psi for a few MONTHS, not minutes, hours or days, MONTHS
New fuel pump may have a bad check valve
Or the FPR may be leaking once fuel pump pressure is removed
The fuel line connected to the FPR is the Return line, fuel flows thru this hose back to the gas tank, you could carefully squeeze the hose to block flow
Then with pressure gauge connected cycle key on and off to get to 40psi, and see if it holds
If so then replace FPR
If it still drops to 0psi then fuel pump check valve is the issue, but........................

Stuck open fuel injector, if you had a leaking fuel injector you would get grey smoke from exhaust on startup which you don't mention, and if started engine would run rich and have low power
And when engine was shut off fuel pressure would drop to 0psi

1997 2.3l does use a timing belt and it can get loose and skip a tooth, this lower compression and that will cause loss of power, engine still runs just no power
You would need to test at least 2 cylinders compression, expected on a 1997 2.3l would be 160psi +
If you are lower than 130psi then belt has skipped


Fuel gauge
Pretty simple system, the sender is in the gas tank hooked to a Float
Sender is a variable resistor, like a light dimmer or volume control
It has a range of 16 to 170 OHMS
16 ohms is empty, float at bottom of gas tank
170 ohms is full, float at top of gas tank

The sender is grounded on frame or in cab near inertia switch for fuel pump, same ground point as fuel pump's ground
Then there is a Yellow wire from the sender to the back of the instrument cluster

OHMs
A short is 0 ohms
No connection is infinite or HIGH ohms

So if Yellow wire was shorted to truck metal somewhere the fuel gauge would show below Empty, under 16 ohms
If yellow wire was not connected then gauge would show above Full, higher than 170 ohms

Also in the back side of the instrument cluster is the Anti-Slosh module, these did had issues, this module prevented the gas gauge from going up and down every time you drove around a corner or hit a bump, as the float in the tank will go up and down as gasoline"sloshes" around in the tank

Gouge: Ford Ranger anti-slosh module

Because the gauge reads FULL all the time I would suspect senders Yellow wire or Ground wire it is unplugged at the top of the gas tank, or didn't make a good connection
Stuck at 1/2 tank reads more like slosh module issue

You can used OHM Meter and a sewing needle
Find the Yellow wire in the frame by gas tank
Use needle to pierce it
Use OHM Meter from yellow wire to ground(frame), should read between 16-170ohms
Rock the truck, OHMs should start going up and down as the gasoline sloshes back and forth
If so connection at top of tank and senders ground are OK

 

 

If bed is off, just take off the return line from the top of the tank and put a bolt in the hose to block it off, and a clamp to hold it, 30psi is ALOT of pressure
Cycle key on and off a few times and see if fuel pressure holds

To test fuel pumps check valve take off the OUT hose from the top of tank and put a short hose on it, and BLOW into the hose, you should not be able to blow into the hose, a check valve prevents pressure from flowing back into the tank

Either spark plug can be remove to test compression in a cylinder, unplug coil packs 3 wire connectors, you want a No Start to test conpression

All fuel injection Computers have a "Clear Flooded Engine" routine
Its a way to shut off the fuel injectors in case engine gets flooded
I use it every morning to pump oil thru my high mile 4.0l before I start it

Turn key on
Press gas pedal down to the floor and HOLD IT DOWN all the way
Turn key to START, Crank engine
It should NOT start, it should not even fire, fuel injectors are OFF

If you release gas pedal injectors will start working again
If engine does fire and RPMs get above 200 then computer will restart injectors

Using this to test for leaking injector:
Cycle key on and off a few times
Then do the above
If a fuel injector is leaking then engine will fire because there is gasoline leaking in

If you want to find out which one then pull out 1 spark plug from each cylinder.
Do the "Clear Flooded Engine" test again, you will see fuel being sprayed out of the spark plug hole of the cylinder with leaking injector, can't miss it

One heads up, for "Clear Flooded Engine" to work, gas pedal MUST give you full throttle(WOT)
So test that first, press gas pedal down to the floor and use a stick against seat or a weight to hold it down all the way
Then go to the engine and see if you can manually open the throttle more, if so you need to fix that

Google: ford ranger throttle cable mod

Pretty simple fix, just a couple of zap straps(wires ties) on the gas pedals throttle cable connection, shortens the stretched cable