Sounds like you are chasing too many tails at once. The first thing you need to determine is if the problem is with the fuel supply/input or the IP/lift pump/filter system. The easiest way to do that is to separate those two elements where the fuel inlet line and the fuel return line connect to the fuel supply side of the system.

Get a couple of 3' sections of clear vinyl tubing I'm pretty sure 3/8" ID will fit, Home depot sells 15' rolls for about $5. Then you get a couple liter empty clean soda bottle or some similar sized plastic container, clear if you can because that will allow you to observe both the rate of fuel return and whether it is a solid stream or filled with bubbles.

Mark the hose and the steel fuel lines so you can get them back to where they belong when your done and separate them where they meet in the engine compartment. Be ready to plug the fuel inlet line because if there is much fuel in the tank it will run out until the level drops low enough. A short 3 inch section of hose that has been plugged with a bolt or even a whittled stick will work fine. Get that apart and close off that line with your plug. Do the same with the fuel return line.

Now you have the fuel supply separated from the engine.

Now you take your clear vinyl hose and using about 3 feet of it connect it to the fuel return outlet up at the secondary filter. Should be a hose bib and a hose clamp attachment there. Then take a second 3 foot section of clear vinyl and attach it to the inlet of the primary filter, should be a hose clamp that attachment there also. Then get your soda bottle or whatever plastic vessel you have that you can squeese into place along side the engine on that IP side area, might need to use a piece of wire, string, duct tape ariound the neck of the bottle to secure it from falling down too far or falling over and spilling. But figure out the how and where placement of it.

With that done pull the bottle out and fill it about 2/3 with fresh diesel, place the bottle into the pre-determined position and secure it from falling or dropping. Place the two clear vinyl hoses, one from the return and the other one to the primary filter, place their ends down into the diesel. You probably will want to use a bit of tape, wire, string to hold the tubing in place at the bottle mouth particularly it the engine starts and runs and you want to drive around the block.

You effectively created a alternative mini fuel system attached to the engine, with that hopefully you can get the car started and determine whether your problem is with the IP/filter/lift pump system or with the fuel delivery system.

If the car starts and runs you should be able to drive around the block (actually a couple miles) with it set up like that. If you find the problems with running and dying that you've described have not been eliminated then the problem is not the fuel supply side of the equation.

But you need to separate the two systems to identify where to focus your efforts. With the bottle of fuel you know that the fuel supply is not in question so it can by seeing if the engine runs and drives eliminate the engine side of the equation.

If you get that done then you can focus the future effort on what needs to happen to get things right.

To answer the specific question you posted, as far as testing the steel underchassis fuel supply line, you plug one end and apply vacuum to the other end, and see if it holds vacuum or not. For your vehicle you would need to clamp off the flexible hose attached to the tank's fuel strainer, remove the hose clamp securing the other end of the flexible hose to the steel underchassis fuel line and separate the two, disconnect the flexible fuel hose between the end of the steel line and the primary filter, then remove the fuel from the steel line by blowing it out using just a couple foot section of hose to attach to the steel line and blow in catching the fuel at the other end, fashion some type of plug and seal one end of the steel line and then apply vacuum to the other end and observe whether it holds the applied vacuum.