Here we go again, car cranks but won't start.
 

I know it's a common issue, but I'll go through the basics anyways.

2000 Alero GLS 3.4L

The car will crank but won't start on it's own. If you give it a little gas it will start but will die when you take your foot off the gas.

I suspected the ignition switch\key. Replaced both and had the same issue. Did a security re-learn, NO security lights.

I checked the codes and found no codes related to the car starting. ( just a code for the oil pressure sensor, that I need to replace.)

I cleared the codes and tried to start the car again and it started, repeatedly with no issues.

Waited a few days and it started every time.

Today I tried putting the original key cylinder back in, and it wouldn't start and has the security light on solid. Tried the relearn and the security light won't go out.

Tried the replacement key cylinder again, no security light. And it still won't start.

Tried the original key again and let the car crank for 10-15 sec and it finally started. With the security light on solid. I'm able to restart the car with the security light on solid.

Checked the fuel pressure and it's at 49lbs. (should be between 50-60lbs I believe)

I suspect it's a failing\weak fuel pump, the battery was just replaced a couple months ago and the fuel pump has never been replaced, I'd know I'm the original owner. I'm really close to just disabling the passlock system.

Anybody have any other ideas?

I'm not sure why you tempted fate by putting the old switch back in again after the problem seemd to have been fixed unless I'm reading your post wrong. I'd disconnect the battery for at least 30-minutes. Put the new switch back in and then reconnect the battery. If the car won't start, do the relearn procedure again. If still no start, check the electrical connections on the BCM for corrosion and also verify fuel pressure is present at the fuel rail with the key on.

When doing the relearn procedure after the car won't start, do not turn off the key after trying to start the engine because the relearn may fail. Also make sure all doors are closed and all electrical devices are turned off. I learned this from Mike (Milzymotorsports) and have experienced it on my own GA which is the same car as Alero.

Clean the maf.

I'd check 2 things...

1. Battery terminals (for acid corrosion, stripped/loose terminals and exposed wiring)
2. Fuel pressure (via the shrader valve in the engine compartment.) While on this, I'd also make sure the fuel pump is priming. No hum when placed on "on" position = no fuel being delivered to the engine

The issue still happened with the new switch and cylinder. I found that it appears to start after letting it crank for 10+ seconds. Which is unusual for the car cause even at 14 years old it usually starts right when you turn the key.

It's a fairly new battery, the connections look good. I tested the fuel pressure in the engine compartment and it reads 49lbs and I always hear the fuel pump priming itself. But I have found it will start after cranking it for 10+ seconds, need to test it a bit more cause the problem seems to come and go.

I would check the regulator also

Could be the idle air control valve slowing/going out