Answering your second question first ...

Unfortunately Kodak doesn't sell Rapid Fixer without the hardener, at least not in the quantities that are convenient for the small darkroom user. This is the reason that I have (reluctantly) stopped buying it. You will most likely have to discard the hardener, and if you do, you need to be very careful because the hardener is a fairly strong, concentrated acid. In fact, the hardener is the reason that you cannot ship the package by air.

The reason that the instructions appear to be somewhat complex is that they incorporate the hardener in the mixing workflow. Technically, you could be really precise and factor out the volume of solution B from the equation, but in that case you would end up with a total volume of 124.5 US fluid ounces of film fixer instead of 128 US fluid ounces (1 gallon) of film fixer. The difference in volume is inconsequential.

So instead of following the included directions, just mix the Part A 1 part to 3 parts water. You don't need to mix the entire bottle at once. If (for example) you want 1 quart (32 US ounces) of working strength film fixer, you can mix 8 ounces of concentrate with 24 ounces of water.

Kodak recommends starting with part of the water, adding the concentrate, and then adding further water to bring the total to the target. That is generally a good idea, because for some chemistry the addition of different constituent parts results in a change in volume.

Hope this helps.