tremendous amount of informationI’m trying to answer se of your questions here:
Glow Plugs
I assume what is glowing red is some indicator on the meter not something else.
Glow plugs wiring sound like it will work but the voltage is too high when in the glow plug switch position. It will burn the glow plugs out. Wiring is a bit odd on the tractor wh done correctly. The wiring for the glow plugs is this: battery to the switch (glow plugs and starter switch) then from switch (in glow plug position) to one side of the glow plug indicator (doesn’t matter which side but call this side A). Then through the indicator that is also a resistor to drop the voltage. From the other side of indicator (side B) the wiring then goes to the glow plugs (top of each glow plug are all connected together by the large solid wires that have no insulation and look like a solid rod. From there the power goes through the glow plug and to ground at the bottom of the glow plug where it screws into the block.
When the switch is turned to the starter position the wiring to the glow plugs changes and goes “around” the glow plug indicator and connects to the side B so does not drop the voltage BUT since the starter is cranking the engine the battery voltage is dropped and the amount of time in the start position is usually short.
i would get three new glow plugs and their copper seals since you said that you have leakage at one. I would get a new glow plug indicator also. You can just run wiring temporarily per the above until you put it back together. The indicator will get hot so you should mount it to something like a board and keep it away from fuels. Get a deep socket that fits the glow plugs. All of these parts are inexpensive.
Decompression
The lever at front of the valve cover should be spring loaded and move to the correct position to run. From recent memory (if sitting in the seat) movement to the left will be decompression position and to the right creates compression. You may be able to rig up a string through a hole on the left side of the tractor to pull it open and just let it slack to close it. The cable and the keeper (that pulls the lever) are also relatively inexpensive and probably available. Your engine is spinning ok but sounds a little slow and will spin mu faster if the decompression knob is working. I assume you know how to use it but in short decompress the spin engine up to speed and the release knob to start. You may want to look into a new starter and see if you can get the gear reduction type that will spin it faster. (There is a thread on here that explains how getting the engine to spin faster got a rebuilt engine to start and run but I can’t locate it. The compression was low since the rings had not seated yet)
Fuel
It looks like you are getting fuel but you may want to get new injectors or have yours rebuilt. You can do some tests but I won’t try to explain them.
You need to stop using starter fluid.
You may want to get new fuel if you are not sure how old the fuel is in the tank.
You can verify the fuel flow at the injector pump inlet to verivy the flow is normal.
Glowing red: correct, a volt meter that glows red if there is power, or green if there ground
That indicator is showing power at the glow plugs when starter switch turned left
Getting those parts: Do you think first I should test the current glow plugs and exhaust other avenues before buying parts that potentially are not needed yet?
Decompression knob: From what I am gathering, I should pull it left (from driver position) when cranking, then when she is starting, push knob right?
Fuel: yes, I should clean the injectors and make sure there is no obstructions
I'll look into how much the injectors are, or look into rebuilding them
Starter fluid: yes i agree
Old fuel: I'm not sure how old it is but I think there's a valve under the tank that I can use to drain it out
"You can verify fuel flow at injection pump to verify flow is normal"
How can I verify this exactly? Is there a test that can to see or am I eyeballing it?