XPS 8700 wont power on, can force start with PSU BIST

Following a power outage, my 8700 wouldn’t boot up.  I've run thru the various checks, hard reset, still not getting a clear indicator. I’m leaning toward a PSU over the Power button and motherboard. all removed items had no effect.  appreciate any insights.

For the BIST, it will not power up on first contact, but will on the 2nd or 3rd touch with solid green light and system fans running at high speed. After unplugging and holding down button, then plugging back in, note the MB's aux power light is on but flickering in synch with a barely imperceptible ticking at a cadence of 5 to 6 ticks per second. still no response to power button no matter how long it’s pressed. hitting BIST again, noting first press. Second or third time will get it to green light and power fans AND the aux light is then solid no flicker or ticking after release. 

Noted that when holding down the BIST the LED on power button glows solid amber. while holding down BIST to keep power, power button still has no effect.

This is where it gets interesting. Replaced cmos battery though original was fine, the bist will do the same no initial response then power system on third press with power button now glowing BLUE. keeping BIST pressed it will boot to bios screen. I set the time and save it, then it will boot to windows just fine ( must keep the PSU BIST held in during all this or it dies as soon as released). interestingly the system fan runs at normal speed when the power button is blue and system is botting normally, unlike other times.

The power button still has no effect to sleep or shutdown, like BIST held in bypasses it.  

was able to select shut down from windows while holding the BIST and it exits windows, but while held in the power button went amber at shutdown and then the chassis fan kicked back up to high speed.

After releasing the BIST, still no response to power button.

hitting bist again,  goes back to amber power button and high speed fans, no booting. it was reacting to initial press for a bit at that point.

To confirm, popped out cmos battery to see if it would replicate the previous bios reset behavior. Sure enough, while holding in BIST had a blue power button, normal fan speed, and able it to boot to windows after existing bios screen. So clearing the bios has an effect. But only way to keep it powered on is keeping the BIST held in.

What the forced power on (holding blis in) suggests to me is the cpu/ mb/ main drive / video card are likely fine.

But not sure why the cmos matters, if its an indicator of mb is having a problem, a wonky power button, or nothing at all. That click / flicker at first replug-in makes me think the psu is not able to supply power and keeps shutting down.  I could swear in the past when it’s first plugged in, it would power up for a couple seconds before shutting down / standby.

btw, I've never had a problem with the power button over the last 11 years.  I’m sure the power coming back on after the power failure could have finally fried something though it was on a surge protector.  Appreciate input of how to tell which parts are the culprit. 

Thanks