Last injector return line with a plug is normal.
I'd replace the filters first. Then try the bottle of fuel in the engine compartment. Put two clear lines in a 16 ox pop bottle, one attached to the return line on the canister fuel filter assembly and the other to the input of the small plastic filter. Fuel should get sucked from the bottle and recirculated back into it. A 16 oz (or so) bottle fits well between the inner fender and a hose on my TD. You could drive it around like that to test it if yours fits the same way.
The return line is the 'fat' cigar hose coming off the canister filter assembly. The incoming line goes directly to the small plastic filter. You'll need some additional hose if you want to switch them, or perhaps you can remove the two hoses where they attach to metal lines.
It might work to blow compressed air through the incoming line back through the tank. It would be even better to extend it, put your mouth on the hose and blow through it to see how much resistance you get. I was having problems in a 72 Chevy wagon dying out and found an additional fuel filter someone had installed in the line by blowing backwards through the fuel line and getting a lot of resistance.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13