This is a model with no T2 chip, so no DFU mode. But kudos for even getting to that point. There's actually a wiki for this exact sort of issue which might help.

But I'll cut to the chase already since you've done a lot of this already. Also. Would strongly recommend getting a USB C multimeter for this sort of issue since they can help a LOT.

Usually in this case, the first thing I would do a "hardware isolation" sort of process to see if it's an issue with parts, or the board itself. Unplug literally everything from the board. Including the SSD. Reconnect only whatever the minimum number of things you'll need to know it's powering on. Probably a screen in this case would be the easiest. Even leave battery disconnected.

Plug in a charger, which could prompt it to power on and see if you get anything on the display. It may be REALLY slow since certain components are not connected and the SMC will accommodate by throttling the bejeezus out of the CPU. That's normal. If you get power, start reconnecting and retesting parts one by one until you find the one that results in no power.

Also, would strongly recommend taking the logic board out and giving the USB C ports a good inspection. Damage there can hide easily and can cause a bunch of weird power on issues if there is damage there.

This could certainly be a screen issue. Lid open or closed detection on this model is done by magnetic sensors on the logic board which are positioned to read magnets in the display assembly. The magnets could be in the wrong location, or the screen you used isn't 100% compatible with your specific model. Sleep/wake issues are common in those cases.

If you have access to a magnet, see if you can locate where on the screen the magnets are mounted (or if they exist). by running the magnet along the left and right edges of the display. There should be one on each side. You can also see if you can get the display to sleep by using the magnet to trigger the sensors on the logic board.

By this you mean it’s swiping through the various screens full of apps without you touching anything?

This is almost always an issue with the touch functions of your screen, which would require a screen replacement, but I might have a look at the iPhone Ghost Touch page to try and cover all your bases. Even just restarting your phone can correct this if it’s running into a software issue.

If you're looking for a separate part like a laptop hard drive for the storage, it doesn't look like this tablet has that. I suspect it probably has the storage soldered onto the motherboard. That doesn't mean data isn't accessible, but it will make it more difficult to get.

Either you'll need to repair the unit so it can power on, assuming it doesn't at the moment, or you'll need to hope for easy storage that can be read off the chip directly. Either way you will likely need skills in soldering to get the data.

Some pictures of the device internals could be helpful in locating the specific chip. Or even removable storage if you're lucky, but I didn't see anything like that during my quick research.

Although you'll find various reports as to who recommends what amongst folks in the MacBook repair community. Yes you can. You'll need an Adapter to convert the m.2 style form factor of the Samsung 980 to the correct pin out for the Mac, but those are quite readily available at various online retailers.

Depending on why you're switching out your drive (more capacity, higher speed ratings, your old drive flat out died, etc...) I might have different recommendations on what you use. But it is definitely possible to do what you asked.

Just for point of comparison there is a list of tested drives and adapters at the macrumors forum which might help if you chose to go in this direction.

Make sure your cable is good (try different cables or power sources) and make sure that the cable sits flush into the port, if not, there may be some debris preventing the cable from makling good contact. Try flipping the charge cable, since it will plug into the iPad in either direction and see if that makes a difference.

Otherwise, short of opening the device, I would say your best bet would be to get your hands on a USB multimeter. You could use the one in the iFixit store, if you charge via a USB A Cable. I personally favor a USB C multimeter since it will connect straight to the charger.

You'll want to see what sort of current it's pulling through the charger at a minimum. That can tell you a lot. If it's not drawing any current, then the charge port may be bad (could mean other things as well, that's just the easy explanation).

Do you have any details about what happened prior to the no power issue? Context can also be helpful.

UPDATE - 10/02/2022 - Now that you've gotten the device open, the spots in red are places on the top of the board that are battery voltage. You'll have to peel up the little sticker there. since the battery connector (labelled here as J8900) contacts are all on the bottom of the board. Anyplace that's grey is ground. I usually just use a shield or screw bracket for that.

Battery voltage should be about 3.7 volts. Now that you've got the thing open, I would just have a look at the USB connector as well. The two metal pins on the side you measured, if you mean these two in the pic? Those will be ground so should be continuous, but won't tell you anything. But you should be able able to tell if there is damage to the pins. Pictures of the iPad internals would be super helpful in all cases.

Well. This sounds like a board level issue for sure.

TP1A is the thermistor (thermal resistor, these are often used for temperature monitoring in electronics) for the rear camera. I am not 100% sure of the others. But I have some speculations based on the schematic. There are 3 other thermistors which are present on the board. One for the “Forehead” which is typically the front facing camera assembly. One for the “Radio” which is related to one of the cellular function chips. And one for the CPU. I am on my iPad so no access to board views. But according to the schematic, all these lines connect into the Power Management chip.

They also all connect to electrical ground on their return line. So I’m wondering if there’s a short to ground somewhere in this net. Pictures of the board itself might be helpful, but I know the vast majority of this board is covered in shields, so it can be hard to get a good look at anything. I will have a look at the boardview for this when I get home.

A few things immediately come to mind. Firstly, it could be a bad replacement battery. It wouldn’t be the first conclusion I’d come to, but definitely a possibility.

More likely in my estimation, is that the battery connector may have been damaged during the replacement process. It’s pretty common for the battery data pins to get bent when you’re trying to isolate the battery from the logic board during the repair. I know may people put some kind of barrier between the battery and the boards to keep it from transmitting power to the board. This is a frequent issue in these circumstances. And causes weird battery behavior because information the battery is sending out, like how charged it is, is not making it to its destination.

Screen will work just fine. Face ID will also work just fine as long as you transfer the front sensor assembly from the old display to the new display.

Programmer comes in to get True Tone to function. If your friend doesn't care about that, should be good to go. You will still get the "Important Display Message" regarding the authenticity of the screen. But everything will still work (aside from True Tone).

Well. First. Thanks for a collection of new panic types to add to my iPhone Kernel Panics page. It's still a bit of a work in progress, but sort of a passion project.

Secondly. The SEP panic. Sounds like bad news. The SEP is the Secure Enclave Processor. Which is basically a part of the CPU Which handles and houses all the secure data on the phone. Things like Face ID or Touch ID are most notably stored here. But that's not all. It's super important for proper iPhone boot and normal function.

I will have to do a bit more research since the other two panics are not ones I recognize. But honestly. This sounds like it is very likely an issue with your board.

In which case, if there is a warranty trying to get in touch with the vendor might be a better bet. But have a look at the iPhone Keeps Restarting page just to make sure you've covered your bases if you want to repair this yourself.