I afraid that you are experiencing a known problem with soaker hoses, uneven water release, more water at the beginning than the end. I am guessing the "pressure regulator" that comes with the kit is to keep the hose from rupturing not to help spread at the flow.

You might want to check out "Rain Bird" or "Netafim" drip lines. They have evenly space drippers that are pressure regulated so that you will get the same rate of water from beginning to end.

I've successfully used soakers for years for my veggie beds.

Excess incoming water pressure will result in uneven output.

It's easy to over-ride the pinhole pressure-regulating device by turning the faucet on too far.

The directions should state the maximum length for a run. Too long results in the same sort of uneven output.

Despite all these excellent answers, there's one that has gone unanswered: If the hose is in no danger of splitting or spraying water (rather than dripping), shouldn't the water pressure to it be the most it can be to allow for the end of the hose to drip water? Or put another way, how does lowering the water pressure solve the problem of getting more dripping from the far end?

It a hard question to answer without experimenting. IF you can lower the pressure without reducing the flow volume than you would get a lower flow rate out of the holes at the beginning of the pipe ( they are not regulated so flow is controlled by pressure) so logically you should get more water further down the pipe. This is assuming that the flow volume is sufficient and is not reduced as well. Pressure regulators are not expensive, why don't you try it.

If I buy a pressure regulator, should it be adjustable or will one factory set (and fixed) at 40-50 be OK?

You want a pressure regulator, fixed of about 20 psi. Should be cheap, in the 5 dollar range. I think that you should remove the flow disk.

If you cut the 100 foot hose in half, you're using 50 foot runs, which are too long. Try to keep your runs 15 feet or less. You should have gotten 4 T connectors and 2 feeder connectors, use them to keep your runs shorter than 50 feet.

Can you drill a little larger hole in the second disk??