How far should the return fuel line protrude into the fuel tank?

Usually if cranks good but won't start, you have a helper crank it while you visually check for spark at the spark plugs. If spark everywhere, use a gage and check proper fuel pressure and check fuel injector pulse. I don't know what testing you have done?
As far as the fuel regulator, it depends? If there is a return line, the pressure regulator should be on the fuel rail. If no return line, with mechanical returnless, the regulator may be in a loop close to gas tank. With electronic returnless, everything is in the gas tank. Sorry, I haven't found any pictures?

Fuel goes through fuel filter in tank to carb to purge bulb and back into tank.

The gas line with the weighted fuel filter on it should lay across the bottom of it (somewhere near the opposite side) and the purge/return line only needs to enter the fuel tank 1/2" to3/4" to work.

There are two fuel lines, a supply and a return. The supply line is the long one and hangs in the tank where it can follow the fuel as the machine is moved around, it often has a weighted filter attached to it. The return line just protrudes into the tank from the carb. To sort out the pipes, fit them and operate the primer bulb. The fuel should be sucked into the long tube and return through the short one. There are special slilcone rubber tubes made especially for these small machines. Normal plastic tubing goes hard, will not move around with the fuel movement and eventually disintergrates.

Hi pwdw71...
You will need to go to the lawnmower/weedeater parts house with a sample of both lines to be able to replace the lines with the right size fuel lines.
Also check/clean/replace the fuel filter inside of the tank.
Also take your Mfg. and model number with you.
When you remove your fuel lines from the carburetor be sure to make a drawing to how the lines are connected to the carburetor.
Normally the big line will be the line the fuel filter is connected to inside of the tank.
The smaller of the two lines is the return to the fuel tank from the carburetor after it is pumped thru and primed the carburetor by the primer bulb.
You will insert the big line into the fuel tank far enough to be able to install the fuel filter.
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There are usually only two fuel lines on a Ryobi motor, a supply and a return line. The supply line is a long one and is designed to move around the tank as the unit is moved around, some hafe a weight with a filter on in the tank. The other line is shorter and just protrudes into the tank, this is the return line. The piping you fit should be the special silicon rubber pipes as these are fuel proof, normal plastic pipes become hard and do not move around in the tank, and eventually crack or disintergrate. Replace the new pipes the same as you remove the old ones, if you go wrong, just change the pipes around on the carb, you will see if you are sucking air with the primer bulb.

the little fuel filter goes in the line to the carb. make sure it will move all over in the tank so it can pick up gas.
the other line is the return line it comes from the primer bulb.
the primer bulb has 2 connection 1 to the carb. and 1 to the gas tank.
it sucks fuel thu the carb. and back to the tank.
if it has a crack or pin hole it might start but won't run.

It should have the little nylon connector on the very end of it at the very bottom of the fuel tank holding it in. You don't have to have the fitting on it. Since it is connected on the purge valve RETURN line, that is all that is necessary..long as it doesn't pull out of the tank...that line staying at bottom of tank keeps more air out of it as it carries old fuel and any air from the carb ahead of newer, fresh fuel entering the carb.

Watch the video, bar far the best method I have found.
Fuel Line Replacement by SmallEng.com
http://www.youtube.com/watch?v=JrXpU70Hmi0
• The hose in the tank with the filter on the free end connects directly to the carburetor input connection.
• If the saw is equipped with a primer bulb, an output line from the carburetor connects to the suction side of the primer (once the bulb is collapsed it draws fuel from the tank through the carburetor to refill as it inflates).
• The pressure side of the primer returns displaced fuel to the tank as it is depressed.
Take lots of notes and a few digital photographs to help with reassembly.

It should be as illustrated on the link.
Typical Fuel System (Primer Bulb and fuel lines may not apply)
http://www.fixya.com/support/r3706813-2_cycle_fuel_system_repair
The line with the filter needs to be long enough to reach all parts of the tank as the saw is moved. The return line is not a critical length probably mid tank. Do you have the filtered hose feeding the primer and not the carburetor? That will cause what you describe. HTH
Lou