To eliminate ground issues for the weak glowing glow plugs. Take a jumper cable from the negative terminal of the battery to a good connection on the engine and watch that glow plug again.

Or if the injector is already back in read the plug voltage with the jumper and without it. A weak ground will be indicated by a higher voltage at the glow plugs with the temporary additional ground.

At the same time try your starter with the temporary additional ground. Is it noticeably faster turning? Make sure your connections with a temporary jumper are really sound at both ends.