Computer will not charge/power; no light on magsafe charger

I cannot, for the life of me, figure out what is wrong my Early 2011 15” MacBook Pro.

I resurrected this computer to function as a Zoom terminal for a school, and it did its job wonderfully all year. Starting a few weeks ago, however, charging began to get erratic. The computer would suddenly turn off, it was reported to me, despite being plugged in. This concerned me but after taking a look at it, it was clear the battery would charge, perhaps partially, and then stop, the light would go out on the magsafe charger unnoticed, and then the computer would operate on battery for however long a charge it had, then, unsurprisingly, it would turn off once it ran out of battery juice.

Swapping around the magsafe charger back to front worked…once. Then it turned off again. Leaving the magsafe charger disconnected and then trying again several hours later worked…once. Changing chargers worked…once. But the problem would almost immediately come back. I’m not sure what I was doing when that light last agreed to come on, but I got the battery about 70% charged before it blinked out again, and that’s the last time I got power into the battery.

I took it home and did the usual: PRAM, SMC reset. I cleaned the terminals on both sides, and I know I’m using a functional charger, and the wall socket has power. Battery is third party, but it is less than a year old and there were no issues with it. It holds a nice long charge (when the computer decides to charge) and I don’t see any real issue with the battery.

Opening up:

1) Disconnecting the battery, as a way to rule out battery issues, then plugging in the magsafe charger. When I did this, the magsafe charger turned green and the computer turned on, and booted properly. I shut it down, disconnected the magsafe charger, and tried putting it back and nothing. It won’t light up again, and certainly won’t start without power.

2) When the battery is connected, the computer runs fine on the battery, and the computer reports no issue. It acts as if the magsafe charger is simply not attached. The LED battery status lights on the side properly light up. I’m counting down from 70% charged and I assume once it runs out, I’m dead in the water, so I’m trying hard not to run this thing on battery while I diagnose.

3) DC-input board. I took a flyer on this one and ordered a new one because it was very inexpensive. I installed it and it made no difference. The computer simply will not recognize the magsafe charger.

Background history on the computer is that it sat unused with a cracked front glass for a lot of years after I got something new, during which time the battery (original) swelled, which I noticed when I unshelved it to bring it back into service last year. I replaced just the front glass because the LCD panel was fine. After replacing the battery, the trackpad remained glitchy but turning off the function where you tap to click fixed that. After that, it was smooth sailing. Ages ago, the bottom RAM slot stopped working, but 8GB in the top slot is more than enough for what I ask of it.

I don’t know what to think now, or what else I can possibly try to do to isolate the problem, but I would very much like to get this computer up and running again. It was, until this issue, a terrific one-job workhorse with a big bright screen.