The 6.2 is easy. Fuel, air, heat and compression.
Since you got rattle with the ether, you have air. Probably compression too.
A gasoline engine needs to spin around 60 rpm with the starter to fire up. Diesel engines need at least 180 rpm. So, yes both batteries load tested and fully charged with a good starter motor is required. That gives the compression.
Fuel on the 6.2 needs to follow a path. Fuel tank to the lift pump. There is a neoprene hose between the tank and the fuel line going along the frame rail. Sometimes a neoprene hose connecting two fuel lines under the passenger seat. Then the S shaped line going from the frame to the lift pump. A pin hole in any of those could introduce air and run the IP dry.
The lift pump sends the fuel through a metal line trough the valley yo a neoprene hose by the #6 intake runner to the fuel filter. A leak there could also cause a dry IP.
Then there is the factory square fuel filter. The base of them is notorious for having the water in fuel sensor on the back of it leaking air into the system but since fuel is larger at the molecule level. No fuel leaking out. Very hard to find. Pulling the pink wire from the IP, putting a clear bottle on the clear bleeder hose on the fuel filter, opening the bleeder valve and then cranking the engine. If air is found, then there is a leak somewhere.
If you do have air with bleeding the filter. Then, the IP is dry. Plug the pink wire back in, plug it in with the key on. You should hear a click. If no click then the on/off solenoid might be bad or the pink wire is not getting voltage. If all is good. About 2 minutes total of cranking will be needed to get fuel to the injectors again. Don’t do it in a single cranking session. Crank for 15-20 seconds, wait 2 minutes, repeat. Pulling the glow plugs speeds this up and lets you see clouds of fuel blowing out when fuel is being injected.
If you never get fuel clouds then something described above is bad. If you get fuel clouds then glow plugs are next.
A 6.2 needs to have 158° inside the cylinder to start. All 8 glow plugs should be tested while you have them out. Here is a video of how to bench test them:
If you left the glow plugs in and didn’t test them on the engine but while cranking notice greasy white smoke coming out the exhaust. That means you have air, fuel and compression but no heat. It might start if you crank long enough in the summer. But batteries and starter damage will result. 8 new glow plugs and a working controller or good glow plug hot wire system are next. Then, it should start.
Or, if all of the above is correct it still might not start due to the injection pump not pushing enough fuel at a high enough pressure. Since you wrote that it failed to start in the winter. I would suggest glow plug testing first.
