Dell E7450 Won’t Boot After Top Lid Swap.

I had a bad touchscreen on my functioning e7450. (glue was leaking around the LCD)

I took the entire lid off a working e7450 (worked, but the BIOS was locked) and installed on mine.

Now mine won’t even boot to the DELL symbol. Tried plugging in an external HDMI monitor just in case, but no difference.

Retraced all my steps to make sure I all connectors/ribbon cables were fully seated. Visually examined everything to see if I’d knocked anything loose. Nothing suspicious found.
Removed power, battery, and CMOS battery. Held Power button for 30 seconds. Powered up only on the power supply. No change.

Sub-note, don’t know if it’s relevant, but I swapped WLAN cards and keyboards too.

Could really use any help out there on what I could have screwed up! Or what steps to use to troubleshoot.